Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
On Tue, Jul 25, 2017 at 02:50:37PM +0200, Jan Kara wrote: > On Tue 25-07-17 14:15:22, Christoph Hellwig wrote: > > On Tue, Jul 25, 2017 at 11:35:08AM +0200, Jan Kara wrote: > > > On Tue 25-07-17 10:01:58, Christoph Hellwig wrote: > > > > On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote: > > > > > I guess it's up to filesystem if it wants to reuse the same spot to > > > > > write > > > > > data or not. I think your assumptions works for ext4 and xfs. I > > > > > wouldn't > > > > > be that sure for btrfs or other filesystems with CoW support. > > > > > > > > Or XFS with reflinks for that matter. Which currently can't be > > > > combined with DAX, but I had a somewhat working version a few month > > > > ago. > > > > > > But in cases like COW when the block mapping changes, the process > > > must run unmap_mapping_range() before installing the new PTE so that all > > > processes mapping this file offset actually refault and see the new > > > mapping. So this would go through pte_none() case. Am I missing something? > > > > Yes, for DAX COW mappings we'd probably need something like this, unlike > > the pagecache COW handling for which only the underlying block change, > > but not the page. > > Right. So again nothing where the WARN_ON should trigger. Yes. I was confused on how COW is handled. Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov
Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
On Tue 25-07-17 14:15:22, Christoph Hellwig wrote: > On Tue, Jul 25, 2017 at 11:35:08AM +0200, Jan Kara wrote: > > On Tue 25-07-17 10:01:58, Christoph Hellwig wrote: > > > On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote: > > > > I guess it's up to filesystem if it wants to reuse the same spot to > > > > write > > > > data or not. I think your assumptions works for ext4 and xfs. I wouldn't > > > > be that sure for btrfs or other filesystems with CoW support. > > > > > > Or XFS with reflinks for that matter. Which currently can't be > > > combined with DAX, but I had a somewhat working version a few month > > > ago. > > > > But in cases like COW when the block mapping changes, the process > > must run unmap_mapping_range() before installing the new PTE so that all > > processes mapping this file offset actually refault and see the new > > mapping. So this would go through pte_none() case. Am I missing something? > > Yes, for DAX COW mappings we'd probably need something like this, unlike > the pagecache COW handling for which only the underlying block change, > but not the page. Right. So again nothing where the WARN_ON should trigger. That being said I don't care about the WARN_ON too deeply but it can help to catch DAX bugs so if we can keep it I'd prefer to do so... Honza -- Jan Kara SUSE Labs, CR
Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
On Tue, Jul 25, 2017 at 11:35:08AM +0200, Jan Kara wrote: > On Tue 25-07-17 10:01:58, Christoph Hellwig wrote: > > On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote: > > > I guess it's up to filesystem if it wants to reuse the same spot to write > > > data or not. I think your assumptions works for ext4 and xfs. I wouldn't > > > be that sure for btrfs or other filesystems with CoW support. > > > > Or XFS with reflinks for that matter. Which currently can't be > > combined with DAX, but I had a somewhat working version a few month > > ago. > > But in cases like COW when the block mapping changes, the process > must run unmap_mapping_range() before installing the new PTE so that all > processes mapping this file offset actually refault and see the new > mapping. So this would go through pte_none() case. Am I missing something? Yes, for DAX COW mappings we'd probably need something like this, unlike the pagecache COW handling for which only the underlying block change, but not the page.
Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
On Tue 25-07-17 10:01:58, Christoph Hellwig wrote: > On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote: > > I guess it's up to filesystem if it wants to reuse the same spot to write > > data or not. I think your assumptions works for ext4 and xfs. I wouldn't > > be that sure for btrfs or other filesystems with CoW support. > > Or XFS with reflinks for that matter. Which currently can't be > combined with DAX, but I had a somewhat working version a few month > ago. But in cases like COW when the block mapping changes, the process must run unmap_mapping_range() before installing the new PTE so that all processes mapping this file offset actually refault and see the new mapping. So this would go through pte_none() case. Am I missing something? Honza -- Jan Kara SUSE Labs, CR
Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote: > I guess it's up to filesystem if it wants to reuse the same spot to write > data or not. I think your assumptions works for ext4 and xfs. I wouldn't > be that sure for btrfs or other filesystems with CoW support. Or XFS with reflinks for that matter. Which currently can't be combined with DAX, but I had a somewhat working version a few month ago.
Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
On Mon, Jul 24, 2017 at 11:06:12AM -0600, Ross Zwisler wrote:
> @@ -1658,14 +1658,35 @@ static int insert_pfn(struct vm_area_struct *vma,
> unsigned long addr,
> if (!pte)
> goto out;
> retval = -EBUSY;
> - if (!pte_none(*pte))
> - goto out_unlock;
> + if (!pte_none(*pte)) {
> + if (mkwrite) {
> + /*
> + * For read faults on private mappings the PFN passed
> + * in may not match the PFN we have mapped if the
> + * mapped PFN is a writeable COW page. In the mkwrite
> + * case we are creating a writable PTE for a shared
> + * mapping and we expect the PFNs to match.
> + */
Can we?
I guess it's up to filesystem if it wants to reuse the same spot to write
data or not. I think your assumptions works for ext4 and xfs. I wouldn't
be that sure for btrfs or other filesystems with CoW support.
--
Kirill A. Shutemov
[PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
To be able to use the common 4k zero page in DAX we need to have our PTE
fault path look more like our PMD fault path where a PTE entry can be
marked as dirty and writeable as it is first inserted rather than waiting
for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
Right now we can rely on having a dax_pfn_mkwrite() call because we can
distinguish between these two cases in do_wp_page():
case 1: 4k zero page => writable DAX storage
case 2: read-only DAX storage => writeable DAX storage
This distinction is made by via vm_normal_page(). vm_normal_page() returns
false for the common 4k zero page, though, just as it does for DAX ptes.
Instead of special casing the DAX + 4k zero page case we will simplify our
DAX PTE page fault sequence so that it matches our DAX PMD sequence, and
get rid of the dax_pfn_mkwrite() helper. We will instead use
dax_iomap_fault() to handle write-protection faults.
This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set insert_pfn()
will do the work that was previously done by wp_page_reuse() as part of the
dax_pfn_mkwrite() call path.
Signed-off-by: Ross Zwisler
Reviewed-by: Jan Kara
---
include/linux/mm.h | 2 ++
mm/memory.c| 50 +++---
2 files changed, 45 insertions(+), 7 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 46b9ac5..483e84c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2293,6 +2293,8 @@ int vm_insert_pfn_prot(struct vm_area_struct *vma,
unsigned long addr,
unsigned long pfn, pgprot_t pgprot);
int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
pfn_t pfn);
+int vm_insert_mixed_mkwrite(struct vm_area_struct *vma, unsigned long addr,
+ pfn_t pfn);
int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned
long len);
diff --git a/mm/memory.c b/mm/memory.c
index 0e517be..b29dd42 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1646,7 +1646,7 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned
long addr,
EXPORT_SYMBOL(vm_insert_page);
static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
- pfn_t pfn, pgprot_t prot)
+ pfn_t pfn, pgprot_t prot, bool mkwrite)
{
struct mm_struct *mm = vma->vm_mm;
int retval;
@@ -1658,14 +1658,35 @@ static int insert_pfn(struct vm_area_struct *vma,
unsigned long addr,
if (!pte)
goto out;
retval = -EBUSY;
- if (!pte_none(*pte))
- goto out_unlock;
+ if (!pte_none(*pte)) {
+ if (mkwrite) {
+ /*
+* For read faults on private mappings the PFN passed
+* in may not match the PFN we have mapped if the
+* mapped PFN is a writeable COW page. In the mkwrite
+* case we are creating a writable PTE for a shared
+* mapping and we expect the PFNs to match.
+*/
+ if (WARN_ON_ONCE(pte_pfn(*pte) != pfn_t_to_pfn(pfn)))
+ goto out_unlock;
+ entry = *pte;
+ goto out_mkwrite;
+ } else
+ goto out_unlock;
+ }
/* Ok, finally just insert the thing.. */
if (pfn_t_devmap(pfn))
entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
else
entry = pte_mkspecial(pfn_t_pte(pfn, prot));
+
+out_mkwrite:
+ if (mkwrite) {
+ entry = pte_mkyoung(entry);
+ entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+ }
+
set_pte_at(mm, addr, pte, entry);
update_mmu_cache(vma, addr, pte); /* XXX: why not for insert_page? */
@@ -1736,14 +1757,15 @@ int vm_insert_pfn_prot(struct vm_area_struct *vma,
unsigned long addr,
track_pfn_insert(vma, &pgprot, __pfn_to_pfn_t(pfn, PFN_DEV));
- ret = insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot);
+ ret = insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot,
+ false);
return ret;
}
EXPORT_SYMBOL(vm_insert_pfn_prot);
-int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
- pfn_t pfn)
+static int __vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
+ pfn_t pfn, bool mkwrite)
{
pgprot_t pgprot = vma->vm_page_prot;
@@ -1772,10 +1794,24 @@ int vm_insert_mixed(struct vm_area_struct *vma,
unsigned long addr,
page = pfn_to_page(pfn_t_to_pfn(pfn));
return insert_page(vma, addr, page, pgprot);
}
- return insert_pfn(vma, addr, pfn, pgprot);
+ return insert_pfn(vma, addr, pfn, pgprot,
