On Sun, 5 Oct 2014, Kirill A. Shutemov wrote: > On Sat, Oct 04, 2014 at 07:48:04PM -0700, David Rientjes wrote: > > If an anonymous mapping is not allowed to fault thp memory and then > > madvise(MADV_HUGEPAGE) is used after fault, khugepaged will never > > collapse this memory into thp memory. > > > > This occurs because the madvise(2) handler for thp, hugepage_advise(), > > clears VM_NOHUGEPAGE on the stack and it isn't stored in vma->vm_flags > > until the final action of madvise_behavior(). This causes the > > khugepaged_enter_vma_merge() to be a no-op in hugepage_advise() when the This should be hugepage_madvise(). > > vma had previously had VM_NOHUGEPAGE set. > > > > Fix this by passing the correct vma flags to the khugepaged mm slot > > handler. There's no chance khugepaged can run on this vma until after > > madvise_behavior() returns since we hold mm->mmap_sem. > > > > It would be possible to clear VM_NOHUGEPAGE directly from vma->vm_flags > > in hugepage_advise(), but I didn't want to introduce special case > > behavior into madvise_behavior(). I think it's best to just let it > > always set vma->vm_flags itself. > > > > Cc: <stable@xxxxxxxxxxxxxxx> > > Reported-by: Suleiman Souhlal <suleiman@xxxxxxxxxx> > > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx> > > Look like rather complex fix for a not that complex bug. > What about untested patch below? > > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > Date: Sun, 5 Oct 2014 21:22:43 +0300 > Subject: [PATCH] thp: fix registering VMA into khugepaged on > madvise(MADV_HUGEPAGE) > > hugepage_madvise() tries to register VMA into khugepaged with > khugepaged_enter_vma_merge() on madvise(MADV_HUGEPAGE). Unfortunately > it's effectevely nop, since khugepaged_enter_vma_merge() rely on > vma->vm_flags which has not yet updated by the time of > hugepage_madvise(). > > Let's move khugepaged_enter_vma_merge() to the end of madvise_behavior(). > Now we also have chance to catch VMAs which become good for THP after > vma_merge(). > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > --- > mm/huge_memory.c | 8 +++----- > mm/madvise.c | 6 ++++++ > 2 files changed, 9 insertions(+), 5 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index f8ffd9412ec5..f84d52158a66 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1966,12 +1966,10 @@ int hugepage_madvise(struct vm_area_struct *vma, > *vm_flags &= ~VM_NOHUGEPAGE; > *vm_flags |= VM_HUGEPAGE; > /* > - * If the vma become good for khugepaged to scan, > - * register it here without waiting a page fault that > - * may not happen any time soon. > + * vma->vm_flags is not yet updated here. madvise_behavior() > + * will take care to register it in khugepaged once flags > + * updated. > */ > - if (unlikely(khugepaged_enter_vma_merge(vma))) > - return -ENOMEM; > break; > case MADV_NOHUGEPAGE: > /* > diff --git a/mm/madvise.c b/mm/madvise.c > index 0938b30da4ab..60effd2c5e9c 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -128,6 +128,12 @@ success: > */ > vma->vm_flags = new_flags; > > + /* > + * If the vma become good for khugepaged to scan, register it here > + * without waiting a page fault that may not happen any time soon. > + */ > + if (unlikely(khugepaged_enter_vma_merge(vma))) > + error = -ENOMEM; > out: > if (error == -ENOMEM) > error = -EAGAIN; I'm pretty sure this won't compile, but I'm also pretty sure it's easy to come up with an madvise() bit for anon vmas that would cause the BUG_ON() to trigger for CONFIG_DEBUG_VM and unnecessarily do alloc_mm_slot() for madvise() calls that aren't MADV_HUGEPAGE with this that go through the madvise_behavior() path, and for that reason it's probably not as extendable as we'd like. I can verify this tomorrow if you'd like. This is the point of the last paragraph of my changelog to isolate all thp behavior changes to MADV_HUGEPAGE and MADV_NOHUGEPAGE in one place as it's currently done and not add any special handling in madvise_behavior(). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>