On Mon, Oct 15, 2018 at 10:18:14AM +0200, Martin Schwidefsky wrote: > On Mon, 15 Oct 2018 09:10:53 +0200 > Christian Borntraeger <borntrae...@de.ibm.com> wrote: > > > On 10/12/2018 03:37 AM, Joel Fernandes (Google) wrote: > > > Android needs to mremap large regions of memory during memory management > > > related operations. The mremap system call can be really slow if THP is > > > not enabled. The bottleneck is move_page_tables, which is copying each > > > pte at a time, and can be really slow across a large map. Turning on THP > > > may not be a viable option, and is not for us. This patch speeds up the > > > performance for non-THP system by copying at the PMD level when possible. > > > > > > The speed up is three orders of magnitude. On a 1GB mremap, the mremap > > > completion times drops from 160-250 millesconds to 380-400 microseconds. > > > > > > Before: > > > Total mremap time for 1GB data: 242321014 nanoseconds. > > > Total mremap time for 1GB data: 196842467 nanoseconds. > > > Total mremap time for 1GB data: 167051162 nanoseconds. > > > > > > After: > > > Total mremap time for 1GB data: 385781 nanoseconds. > > > Total mremap time for 1GB data: 388959 nanoseconds. > > > Total mremap time for 1GB data: 402813 nanoseconds. > > > > > > Incase THP is enabled, the optimization is skipped. I also flush the > > > tlb every time we do this optimization since I couldn't find a way to > > > determine if the low-level PTEs are dirty. It is seen that the cost of > > > doing so is not much compared the improvement, on both x86-64 and arm64. > > > > > > Cc: minc...@kernel.org > > > Cc: pan...@google.com > > > Cc: hu...@google.com > > > Cc: lokeshgi...@google.com > > > Cc: dan...@google.com > > > Cc: mho...@kernel.org > > > Cc: kir...@shutemov.name > > > Cc: a...@linux-foundation.org > > > Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org> > > > --- > > > mm/mremap.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > 1 file changed, 62 insertions(+) > > > > > > diff --git a/mm/mremap.c b/mm/mremap.c > > > index 9e68a02a52b1..d82c485822ef 100644 > > > --- a/mm/mremap.c > > > +++ b/mm/mremap.c > > > @@ -191,6 +191,54 @@ static void move_ptes(struct vm_area_struct *vma, > > > pmd_t *old_pmd, > > > drop_rmap_locks(vma); > > > } > > > > > > +static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long > > > old_addr, > > > + unsigned long new_addr, unsigned long old_end, > > > + pmd_t *old_pmd, pmd_t *new_pmd, bool *need_flush) > > > +{ > > > + spinlock_t *old_ptl, *new_ptl; > > > + struct mm_struct *mm = vma->vm_mm; > > > + > > > + if ((old_addr & ~PMD_MASK) || (new_addr & ~PMD_MASK) > > > + || old_end - old_addr < PMD_SIZE) > > > + return false; > > > + > > > + /* > > > + * The destination pmd shouldn't be established, free_pgtables() > > > + * should have release it. > > > + */ > > > + if (WARN_ON(!pmd_none(*new_pmd))) > > > + return false; > > > + > > > + /* > > > + * We don't have to worry about the ordering of src and dst > > > + * ptlocks because exclusive mmap_sem prevents deadlock. > > > + */ > > > + old_ptl = pmd_lock(vma->vm_mm, old_pmd); > > > + if (old_ptl) { > > > + pmd_t pmd; > > > + > > > + new_ptl = pmd_lockptr(mm, new_pmd); > > > + if (new_ptl != old_ptl) > > > + spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > > > + > > > + /* Clear the pmd */ > > > + pmd = *old_pmd; > > > + pmd_clear(old_pmd); > > > > Adding Martin Schwidefsky. > > Is this mapping maybe still in use on other CPUs? If yes, I think for > > s390 we need to flush here as well (in other word we might need to introduce > > pmd_clear_flush). On s390 you have to use instructions like CRDTE,IPTE or > > IDTE > > to modify page table entries that are still in use. Otherwise you can get a > > delayed access exception which is - in contrast to page faults - not > > recoverable. > > Just clearing an active pmd would be broken for s390. We need the equivalent > of the ptep_get_and_clear() function for pmds. For s390 this function would > look like this: > > static inline pte_t pmdp_get_and_clear(struct mm_struct *mm, > unsigned long addr, pmd_t *pmdp) > { > return pmdp_xchg_lazy(mm, addr, pmdp, __pmd(_SEGMENT_ENTRY_INVALID)); > } > > Just like pmdp_huge_get_and_clear() in fact.
I agree architecture like s390 may need additional explicit instructions to avoid any unrecoverable failure. So the good news is in my last patch I sent, I have put this behind an architecture flag (HAVE_MOVE_PMD), so we don't have to enable it with architectures that cannot handle it: https://www.spinics.net/lists/linux-mm/msg163621.html Also we are triggering this optimization only if the page is not a transparent huge page by calling pmd_trans_huge(). For regular pages, it should be safe to not do the atomic get_and_clear AIUI because Linux doesn't use any bits from the PMD like the dirty bit if THP is not in use (and the processors that I saw (not s390) should not storing anything in the bits anyway when the page is not a huge page. I have gone through various scenarios and read both arm 32-bit and 64-bit and x86 64-bit manuals, and I believe it to be safe. For s390, lets not set the HAVE_MOVE_PMD flag. Does that work for you? > > > + > > > + VM_BUG_ON(!pmd_none(*new_pmd)); > > > + > > > + /* Set the new pmd */ > > > + set_pmd_at(mm, new_addr, new_pmd, pmd); > > > + if (new_ptl != old_ptl) > > > + spin_unlock(new_ptl); > > > + spin_unlock(old_ptl); > > > + > > > + *need_flush = true; > > > + return true; > > > + } > > > + return false; > > > +} > > > + > > So the idea is to move the pmd entry to the new location, dragging > the whole pte table to a new location with a different address. > I wonder if that is safe in regard to get_user_pages_fast(). Could you elaborate why you feel it may not be? Are you concerned that the PMD moving interferes with the page walk? Incase the tree changes during page-walking, the number of pages pinned by get_user_pages_fast may be less than the number requested. In this case, get_user_pages_fast would fall back to the slow path which should be synchronized with the mremap by courtesy of the mm->mmap_sem. But please let me know the scenario you have in mind and if I missed something. thanks, - Joel