Re: [Nouveau] [PATCH v8 3/8] mm/rmap: Split try_to_munlock from try_to_unmap
On Friday, 21 May 2021 6:24:27 AM AEST Liam Howlett wrote:
[...]
> > > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > > index 977e70803ed8..f09d522725b9 100644
> > > > --- a/mm/rmap.c
> > > > +++ b/mm/rmap.c
> > > > @@ -1405,10 +1405,6 @@ static bool try_to_unmap_one(struct page *page,
> > > > struct vm_area_struct *vma,>
> > > >
> > > > struct mmu_notifier_range range;
> > > > enum ttu_flags flags = (enum ttu_flags)(long)arg;
> > > >
> > > > - /* munlock has nothing to gain from examining un-locked vmas */
> > > > - if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
> > > > - return true;
> > > > -
> > > >
> > > > if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
> > > >
> > > > is_zone_device_page(page) && !is_device_private_page(page))
> > > >
> > > > return true;
> > > >
> > > > @@ -1469,8 +1465,6 @@ static bool try_to_unmap_one(struct page *page,
> > > > struct vm_area_struct *vma,>
> > > >
> > > > page_vma_mapped_walk_done();
> > > > break;
> > > >
> > > > }
> > > >
> > > > - if (flags & TTU_MUNLOCK)
> > > > - continue;
> > > >
> > > > }
> > > >
> > > > /* Unexpected PMD-mapped THP? */
> > > >
> > > > @@ -1784,8 +1778,39 @@ bool try_to_unmap(struct page *page, enum
> > > > ttu_flags
> > > > flags)>
> > > >
> > > > return !page_mapcount(page) ? true : false;
> > > >
> > > > }
> > >
> > > Please add a comment here, especially around locking.
>
> Did you miss this comment? I think the name confusion alone means this
> needs some documentation. It's also worth mentioning arg is unused.
Ack. Was meant to come back to that after discussing some of the locking
questions below. The other side effect of splitting this code out is it leaves
space for more specific documentation which is only a good thing. I will try
and summarise some of the discussion below into a comment here.
> > > > +static bool page_mlock_one(struct page *page, struct vm_area_struct
> > > > *vma,
> > > > + unsigned long address, void *arg)
> > > > +{
> > > > + struct page_vma_mapped_walk pvmw = {
> > > > + .page = page,
> > > > + .vma = vma,
> > > > + .address = address,
> > > > + };
> > > > +
> > > > + /* munlock has nothing to gain from examining un-locked vmas */
> > > > + if (!(vma->vm_flags & VM_LOCKED))
> > > > + return true;
> > >
> > > The logic here doesn't make sense. You called page_mlock_one() on a VMA
> > > that isn't locked and it returns true? Is this a check to see if the
> > > VMA has zero mlock'ed pages?
> >
> > I'm pretty sure the logic is correct. This is used for an rmap_walk, so we
> > return true to continue the page table scan to see if other VMAs have the
> > page locked.
>
> yes, sorry. The logic is correct but doesn't read as though it does.
> I cannot see what is going on easily and there are no comments stating
> what is happening.
Thanks for confirming. The documentation in Documentation/vm/unevictable-
lru.rst is helpful for higher level context but I will put some comments here
around the logic.
> > > > +
> > > > + while (page_vma_mapped_walk()) {
> > > > + /* PTE-mapped THP are never mlocked */
> > > > + if (!PageTransCompound(page)) {
> > > > + /*
> > > > + * Holding pte lock, we do *not* need
> > > > + * mmap_lock here
> > > > + */
> > >
> > > Are you sure? I think you at least need to hold the mmap lock for
> > > reading to ensure there's no race here? mlock_vma_page() eludes to such
> > > a scenario when lazy mlocking.
> >
> > Not really. I don't claim to be an mlock expert but as this is a clean-up
> > for try_to_unmap() the intent was to not change existing behaviour.
> >
> > However presenting the function in this simplified form did raise this and
> > some other questions during previous reviews - see
> > https://lore.kernel.org/
> > dri-devel/20210331115746.ga1463...@nvidia.com/ for the previous
> > discussion.
>
> From what I can see, at least the following paths have mmap_lock held
> for writing:
>
> munlock_vma_pages_range() from __do_munmap()
> munlokc_vma_pages_range() from remap_file_pages()
>
> > To answer the questions around locking though I did do some git sha1
> > mining. The best explanation seemed to be contained in
> > https://git.kernel.org/pub/scm/
> > linux/kernel/git/torvalds/linux.git/commit/?
> > id=b87537d9e2feb30f6a962f27eb32768682698d3b from Hugh (whom I've added
> > again here in case he can help answer some of these).
>
> Thanks for the pointer. That race doesn't make the lock unnecessary.
> It is the exception to the rule because the
Re: [Nouveau] [PATCH v8 3/8] mm/rmap: Split try_to_munlock from try_to_unmap
* Alistair Popple [210519 08:38]: > On Wednesday, 19 May 2021 6:04:51 AM AEST Liam Howlett wrote: > > External email: Use caution opening links or attachments > > > > * Alistair Popple [210407 04:43]: > > > The behaviour of try_to_unmap_one() is difficult to follow because it > > > performs different operations based on a fairly large set of flags used > > > in different combinations. > > > > > > TTU_MUNLOCK is one such flag. However it is exclusively used by > > > try_to_munlock() which specifies no other flags. Therefore rather than > > > overload try_to_unmap_one() with unrelated behaviour split this out into > > > it's own function and remove the flag. > > > > > > Signed-off-by: Alistair Popple > > > Reviewed-by: Ralph Campbell > > > Reviewed-by: Christoph Hellwig > > > > > > --- > > > > > > v8: > > > * Renamed try_to_munlock to page_mlock to better reflect what the > > > > > > function actually does. > > > > > > * Removed the TODO from the documentation that this patch addresses. > > > > > > v7: > > > * Added Christoph's Reviewed-by > > > > > > v4: > > > * Removed redundant check for VM_LOCKED > > > --- > > > > > > Documentation/vm/unevictable-lru.rst | 33 --- > > > include/linux/rmap.h | 3 +- > > > mm/mlock.c | 10 +++--- > > > mm/rmap.c| 48 +--- > > > 4 files changed, 55 insertions(+), 39 deletions(-) > > > > > > diff --git a/Documentation/vm/unevictable-lru.rst > > > b/Documentation/vm/unevictable-lru.rst index 0e1490524f53..eae3af17f2d9 > > > 100644 > > > --- a/Documentation/vm/unevictable-lru.rst > > > +++ b/Documentation/vm/unevictable-lru.rst > > > @@ -389,14 +389,14 @@ mlocked, munlock_vma_page() updates that zone > > > statistics for the number of> > > > mlocked pages. Note, however, that at this point we haven't checked > > > whether the page is mapped by other VM_LOCKED VMAs. > > > > > > -We can't call try_to_munlock(), the function that walks the reverse map > > > to > > > +We can't call page_mlock(), the function that walks the reverse map to > > > > > > check for other VM_LOCKED VMAs, without first isolating the page from the > > > LRU.> > > > -try_to_munlock() is a variant of try_to_unmap() and thus requires that > > > the page +page_mlock() is a variant of try_to_unmap() and thus requires > > > that the page> > > > not be on an LRU list [more on these below]. However, the call to > > > > > > -isolate_lru_page() could fail, in which case we couldn't > > > try_to_munlock(). So, +isolate_lru_page() could fail, in which case we > > > can't call page_mlock(). So,> > > > we go ahead and clear PG_mlocked up front, as this might be the only > > > chance we> > > > -have. If we can successfully isolate the page, we go ahead and > > > -try_to_munlock(), which will restore the PG_mlocked flag and update the > > > zone +have. If we can successfully isolate the page, we go ahead and > > > call +page_mlock(), which will restore the PG_mlocked flag and update the > > > zone> > > > page statistics if it finds another VMA holding the page mlocked. If we > > > fail to isolate the page, we'll have left a potentially mlocked page on > > > the LRU. This is fine, because we'll catch it later if and if vmscan > > > tries to reclaim> > > > @@ -545,31 +545,24 @@ munlock or munmap system calls, mm teardown > > > (munlock_vma_pages_all), reclaim,> > > > holepunching, and truncation of file pages and their anonymous COWed > > > pages. > > > > > > -try_to_munlock() Reverse Map Scan > > > +page_mlock() Reverse Map Scan > > > > > > - > > > > > > -.. warning:: > > > - [!] TODO/FIXME: a better name might be page_mlocked() - analogous to > > > the - page_referenced() reverse map walker. > > > - > > > > > > When munlock_vma_page() [see section :ref:`munlock()/munlockall() System > > > Call Handling ` above] tries to munlock a > > > page, it needs to determine whether or not the page is mapped by any > > > VM_LOCKED VMA without actually attempting to unmap all PTEs from the > > > page. For this purpose, the unevictable/mlock infrastructure > > > > > > -introduced a variant of try_to_unmap() called try_to_munlock(). > > > +introduced a variant of try_to_unmap() called page_mlock(). > > > > > > -try_to_munlock() calls the same functions as try_to_unmap() for anonymous > > > and -mapped file and KSM pages with a flag argument specifying unlock > > > versus unmap -processing. Again, these functions walk the respective > > > reverse maps looking -for VM_LOCKED VMAs. When such a VMA is found, as > > > in the try_to_unmap() case, -the functions mlock the page via > > > mlock_vma_page() and return SWAP_MLOCK. This -undoes the pre-clearing of > > > the page's PG_mlocked done by munlock_vma_page. +page_mlock() walks the > > > respective reverse maps looking for VM_LOCKED VMAs. When +such a VMA is > > > found the page
Re: [Nouveau] [PATCH v8 3/8] mm/rmap: Split try_to_munlock from try_to_unmap
On Wednesday, 19 May 2021 6:04:51 AM AEST Liam Howlett wrote: > External email: Use caution opening links or attachments > > * Alistair Popple [210407 04:43]: > > The behaviour of try_to_unmap_one() is difficult to follow because it > > performs different operations based on a fairly large set of flags used > > in different combinations. > > > > TTU_MUNLOCK is one such flag. However it is exclusively used by > > try_to_munlock() which specifies no other flags. Therefore rather than > > overload try_to_unmap_one() with unrelated behaviour split this out into > > it's own function and remove the flag. > > > > Signed-off-by: Alistair Popple > > Reviewed-by: Ralph Campbell > > Reviewed-by: Christoph Hellwig > > > > --- > > > > v8: > > * Renamed try_to_munlock to page_mlock to better reflect what the > > > > function actually does. > > > > * Removed the TODO from the documentation that this patch addresses. > > > > v7: > > * Added Christoph's Reviewed-by > > > > v4: > > * Removed redundant check for VM_LOCKED > > --- > > > > Documentation/vm/unevictable-lru.rst | 33 --- > > include/linux/rmap.h | 3 +- > > mm/mlock.c | 10 +++--- > > mm/rmap.c| 48 +--- > > 4 files changed, 55 insertions(+), 39 deletions(-) > > > > diff --git a/Documentation/vm/unevictable-lru.rst > > b/Documentation/vm/unevictable-lru.rst index 0e1490524f53..eae3af17f2d9 > > 100644 > > --- a/Documentation/vm/unevictable-lru.rst > > +++ b/Documentation/vm/unevictable-lru.rst > > @@ -389,14 +389,14 @@ mlocked, munlock_vma_page() updates that zone > > statistics for the number of> > > mlocked pages. Note, however, that at this point we haven't checked > > whether the page is mapped by other VM_LOCKED VMAs. > > > > -We can't call try_to_munlock(), the function that walks the reverse map > > to > > +We can't call page_mlock(), the function that walks the reverse map to > > > > check for other VM_LOCKED VMAs, without first isolating the page from the > > LRU.> > > -try_to_munlock() is a variant of try_to_unmap() and thus requires that > > the page +page_mlock() is a variant of try_to_unmap() and thus requires > > that the page> > > not be on an LRU list [more on these below]. However, the call to > > > > -isolate_lru_page() could fail, in which case we couldn't > > try_to_munlock(). So, +isolate_lru_page() could fail, in which case we > > can't call page_mlock(). So,> > > we go ahead and clear PG_mlocked up front, as this might be the only > > chance we> > > -have. If we can successfully isolate the page, we go ahead and > > -try_to_munlock(), which will restore the PG_mlocked flag and update the > > zone +have. If we can successfully isolate the page, we go ahead and > > call +page_mlock(), which will restore the PG_mlocked flag and update the > > zone> > > page statistics if it finds another VMA holding the page mlocked. If we > > fail to isolate the page, we'll have left a potentially mlocked page on > > the LRU. This is fine, because we'll catch it later if and if vmscan > > tries to reclaim> > > @@ -545,31 +545,24 @@ munlock or munmap system calls, mm teardown > > (munlock_vma_pages_all), reclaim,> > > holepunching, and truncation of file pages and their anonymous COWed > > pages. > > > > -try_to_munlock() Reverse Map Scan > > +page_mlock() Reverse Map Scan > > > > - > > > > -.. warning:: > > - [!] TODO/FIXME: a better name might be page_mlocked() - analogous to > > the - page_referenced() reverse map walker. > > - > > > > When munlock_vma_page() [see section :ref:`munlock()/munlockall() System > > Call Handling ` above] tries to munlock a > > page, it needs to determine whether or not the page is mapped by any > > VM_LOCKED VMA without actually attempting to unmap all PTEs from the > > page. For this purpose, the unevictable/mlock infrastructure > > > > -introduced a variant of try_to_unmap() called try_to_munlock(). > > +introduced a variant of try_to_unmap() called page_mlock(). > > > > -try_to_munlock() calls the same functions as try_to_unmap() for anonymous > > and -mapped file and KSM pages with a flag argument specifying unlock > > versus unmap -processing. Again, these functions walk the respective > > reverse maps looking -for VM_LOCKED VMAs. When such a VMA is found, as > > in the try_to_unmap() case, -the functions mlock the page via > > mlock_vma_page() and return SWAP_MLOCK. This -undoes the pre-clearing of > > the page's PG_mlocked done by munlock_vma_page. +page_mlock() walks the > > respective reverse maps looking for VM_LOCKED VMAs. When +such a VMA is > > found the page is mlocked via mlock_vma_page(). This undoes the > > +pre-clearing of the page's PG_mlocked done by munlock_vma_page. > > > > -Note that try_to_munlock()'s reverse map walk must visit every VMA in a > > page's +Note that page_mlock()'s reverse map
Re: [Nouveau] [PATCH v8 3/8] mm/rmap: Split try_to_munlock from try_to_unmap
* Alistair Popple [210407 04:43]: > The behaviour of try_to_unmap_one() is difficult to follow because it > performs different operations based on a fairly large set of flags used > in different combinations. > > TTU_MUNLOCK is one such flag. However it is exclusively used by > try_to_munlock() which specifies no other flags. Therefore rather than > overload try_to_unmap_one() with unrelated behaviour split this out into > it's own function and remove the flag. > > Signed-off-by: Alistair Popple > Reviewed-by: Ralph Campbell > Reviewed-by: Christoph Hellwig > > --- > > v8: > * Renamed try_to_munlock to page_mlock to better reflect what the > function actually does. > * Removed the TODO from the documentation that this patch addresses. > > v7: > * Added Christoph's Reviewed-by > > v4: > * Removed redundant check for VM_LOCKED > --- > Documentation/vm/unevictable-lru.rst | 33 --- > include/linux/rmap.h | 3 +- > mm/mlock.c | 10 +++--- > mm/rmap.c| 48 +--- > 4 files changed, 55 insertions(+), 39 deletions(-) > > diff --git a/Documentation/vm/unevictable-lru.rst > b/Documentation/vm/unevictable-lru.rst > index 0e1490524f53..eae3af17f2d9 100644 > --- a/Documentation/vm/unevictable-lru.rst > +++ b/Documentation/vm/unevictable-lru.rst > @@ -389,14 +389,14 @@ mlocked, munlock_vma_page() updates that zone > statistics for the number of > mlocked pages. Note, however, that at this point we haven't checked whether > the page is mapped by other VM_LOCKED VMAs. > > -We can't call try_to_munlock(), the function that walks the reverse map to > +We can't call page_mlock(), the function that walks the reverse map to > check for other VM_LOCKED VMAs, without first isolating the page from the > LRU. > -try_to_munlock() is a variant of try_to_unmap() and thus requires that the > page > +page_mlock() is a variant of try_to_unmap() and thus requires that the page > not be on an LRU list [more on these below]. However, the call to > -isolate_lru_page() could fail, in which case we couldn't try_to_munlock(). > So, > +isolate_lru_page() could fail, in which case we can't call page_mlock(). So, > we go ahead and clear PG_mlocked up front, as this might be the only chance > we > -have. If we can successfully isolate the page, we go ahead and > -try_to_munlock(), which will restore the PG_mlocked flag and update the zone > +have. If we can successfully isolate the page, we go ahead and call > +page_mlock(), which will restore the PG_mlocked flag and update the zone > page statistics if it finds another VMA holding the page mlocked. If we fail > to isolate the page, we'll have left a potentially mlocked page on the LRU. > This is fine, because we'll catch it later if and if vmscan tries to reclaim > @@ -545,31 +545,24 @@ munlock or munmap system calls, mm teardown > (munlock_vma_pages_all), reclaim, > holepunching, and truncation of file pages and their anonymous COWed pages. > > > -try_to_munlock() Reverse Map Scan > +page_mlock() Reverse Map Scan > - > > -.. warning:: > - [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the > - page_referenced() reverse map walker. > - > When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call > Handling ` above] tries to munlock a > page, it needs to determine whether or not the page is mapped by any > VM_LOCKED VMA without actually attempting to unmap all PTEs from the > page. For this purpose, the unevictable/mlock infrastructure > -introduced a variant of try_to_unmap() called try_to_munlock(). > +introduced a variant of try_to_unmap() called page_mlock(). > > -try_to_munlock() calls the same functions as try_to_unmap() for anonymous and > -mapped file and KSM pages with a flag argument specifying unlock versus unmap > -processing. Again, these functions walk the respective reverse maps looking > -for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case, > -the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK. > This > -undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page. > +page_mlock() walks the respective reverse maps looking for VM_LOCKED VMAs. > When > +such a VMA is found the page is mlocked via mlock_vma_page(). This undoes the > +pre-clearing of the page's PG_mlocked done by munlock_vma_page. > > -Note that try_to_munlock()'s reverse map walk must visit every VMA in a > page's > +Note that page_mlock()'s reverse map walk must visit every VMA in a page's > reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA. > However, the scan can terminate when it encounters a VM_LOCKED VMA. > -Although try_to_munlock() might be called a great many times when munlocking > a > +Although page_mlock() might be called a great many times when munlocking a > large region or
[Nouveau] [PATCH v8 3/8] mm/rmap: Split try_to_munlock from try_to_unmap
The behaviour of try_to_unmap_one() is difficult to follow because it performs different operations based on a fairly large set of flags used in different combinations. TTU_MUNLOCK is one such flag. However it is exclusively used by try_to_munlock() which specifies no other flags. Therefore rather than overload try_to_unmap_one() with unrelated behaviour split this out into it's own function and remove the flag. Signed-off-by: Alistair Popple Reviewed-by: Ralph Campbell Reviewed-by: Christoph Hellwig --- v8: * Renamed try_to_munlock to page_mlock to better reflect what the function actually does. * Removed the TODO from the documentation that this patch addresses. v7: * Added Christoph's Reviewed-by v4: * Removed redundant check for VM_LOCKED --- Documentation/vm/unevictable-lru.rst | 33 --- include/linux/rmap.h | 3 +- mm/mlock.c | 10 +++--- mm/rmap.c| 48 +--- 4 files changed, 55 insertions(+), 39 deletions(-) diff --git a/Documentation/vm/unevictable-lru.rst b/Documentation/vm/unevictable-lru.rst index 0e1490524f53..eae3af17f2d9 100644 --- a/Documentation/vm/unevictable-lru.rst +++ b/Documentation/vm/unevictable-lru.rst @@ -389,14 +389,14 @@ mlocked, munlock_vma_page() updates that zone statistics for the number of mlocked pages. Note, however, that at this point we haven't checked whether the page is mapped by other VM_LOCKED VMAs. -We can't call try_to_munlock(), the function that walks the reverse map to +We can't call page_mlock(), the function that walks the reverse map to check for other VM_LOCKED VMAs, without first isolating the page from the LRU. -try_to_munlock() is a variant of try_to_unmap() and thus requires that the page +page_mlock() is a variant of try_to_unmap() and thus requires that the page not be on an LRU list [more on these below]. However, the call to -isolate_lru_page() could fail, in which case we couldn't try_to_munlock(). So, +isolate_lru_page() could fail, in which case we can't call page_mlock(). So, we go ahead and clear PG_mlocked up front, as this might be the only chance we -have. If we can successfully isolate the page, we go ahead and -try_to_munlock(), which will restore the PG_mlocked flag and update the zone +have. If we can successfully isolate the page, we go ahead and call +page_mlock(), which will restore the PG_mlocked flag and update the zone page statistics if it finds another VMA holding the page mlocked. If we fail to isolate the page, we'll have left a potentially mlocked page on the LRU. This is fine, because we'll catch it later if and if vmscan tries to reclaim @@ -545,31 +545,24 @@ munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim, holepunching, and truncation of file pages and their anonymous COWed pages. -try_to_munlock() Reverse Map Scan +page_mlock() Reverse Map Scan - -.. warning:: - [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the - page_referenced() reverse map walker. - When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call Handling ` above] tries to munlock a page, it needs to determine whether or not the page is mapped by any VM_LOCKED VMA without actually attempting to unmap all PTEs from the page. For this purpose, the unevictable/mlock infrastructure -introduced a variant of try_to_unmap() called try_to_munlock(). +introduced a variant of try_to_unmap() called page_mlock(). -try_to_munlock() calls the same functions as try_to_unmap() for anonymous and -mapped file and KSM pages with a flag argument specifying unlock versus unmap -processing. Again, these functions walk the respective reverse maps looking -for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case, -the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK. This -undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page. +page_mlock() walks the respective reverse maps looking for VM_LOCKED VMAs. When +such a VMA is found the page is mlocked via mlock_vma_page(). This undoes the +pre-clearing of the page's PG_mlocked done by munlock_vma_page. -Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's +Note that page_mlock()'s reverse map walk must visit every VMA in a page's reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA. However, the scan can terminate when it encounters a VM_LOCKED VMA. -Although try_to_munlock() might be called a great many times when munlocking a +Although page_mlock() might be called a great many times when munlocking a large region or tearing down a large address space that has been mlocked via mlockall(), overall this is a fairly rare event. @@ -602,7 +595,7 @@ inactive lists to the appropriate node's unevictable list. shrink_inactive_list() should only see SHM_LOCK'd pages
