Messages in this thread Patch in this message | | | From | Alex Thorlton <> | Subject | [BUG] mm: thp: hugepage_vma_check has a blind spot | Date | Tue, 21 Jan 2014 17:07:49 -0600 |
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hugepage_vma_check is called during khugepaged_scan_mm_slot to ensure that khugepaged doesn't try to allocate THPs in vmas where they are disallowed, either due to THPs being disabled system-wide, or through MADV_NOHUGEPAGE.
The logic that hugepage_vma_check uses doesn't seem to cover all cases, in my opinion. Looking at the original code:
if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) || (vma->vm_flags & VM_NOHUGEPAGE))
We can see that it's possible to have THP disabled system-wide, but still receive THPs in this vma. It seems that it's assumed that just because khugepaged_always == false, TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG must be set, which is not the case. We could have VM_HUGEPAGE set, but have THP set to "never" system-wide, in which case, the condition presented in the if will evaluate to false, and (provided the other checks pass) we can end up giving out a THP even though the behavior is set to "never."
While we do properly check these flags in khugepaged_has_work, it looks like it's possible to sleep after we check khugepaged_hask_work, but before hugepage_vma_check, during which time, hugepages could have been disabled system-wide, in which case, we could hand out THPs when we shouldn't be.
This small fix makes hugepage_vma_check work more like transparent_hugepage_enabled, checking if THPs are set to "always" system-wide, then checking if THPs are set to "madvise," as well as making sure that VM_HUGEPAGE is set for this vma.
Signed-off-by: Alex Thorlton <athorlton@sgi.com> Reported-by: Alex Thorlton <athorlton@sgi.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Mel Gorman <mgorman@suse.de> Cc: Rik van Riel <riel@redhat.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org
--- mm/huge_memory.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 95d1acb..f62fba9 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2394,7 +2394,8 @@ static struct page static bool hugepage_vma_check(struct vm_area_struct *vma) { - if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) || + if ((!khugepaged_always() || + (!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_req_madv())) || (vma->vm_flags & VM_NOHUGEPAGE)) return false; -- 1.7.12.4
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