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mm: page_mkclean vs MADV_DONTNEED race

PATCH In review Aneesh Kumar K.V 2 participants 3 messages Mar 21, 2019 – Mar 22, 2019

Subsystems (4)
Memory management VFS MM rmap Filesystem direct access (DAX)
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Aneesh Kumar K.V

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#0 lore raw

PATCH 1/1 mm: page_mkclean vs MADV_DONTNEED race

MADV_DONTNEED is handled with mmap_sem taken in read mode. We call page_mkclean without holding mmap_sem.

MADV_DONTNEED implies that pages in the region are unmapped and subsequent access to the pages in that range is handled as a new page fault. This implies that if we don't have parallel access to the region when MADV_DONTNEED is run we expect those range to be unallocated.

w.r.t page_mkclean we need to make sure that we don't break the MADV_DONTNEED semantics. MADV_DONTNEED check for pmd_none without holding pmd_lock. This implies we skip the pmd if we temporarily mark pmd none. Avoid doing that while marking the page clean.

Keep the sequence same for dax too even though we don't support MADV_DONTNEED for dax mapping

Signed-off-by Aneesh Kumar K.V <[email protected]>
---
 fs/dax.c  | 2 +-
 mm/rmap.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)
fs/dax.c +1 -1
index 01bfb2ac34f9..697bc2f59b90 100644
@@ -814,7 +814,7 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
 				goto unlock_pmd;
 
 			flush_cache_page(vma, address, pfn);
-			pmd = pmdp_huge_clear_flush(vma, address, pmdp);
+			pmd = pmdp_invalidate(vma, address, pmdp);
 			pmd = pmd_wrprotect(pmd);
 			pmd = pmd_mkclean(pmd);
 			set_pmd_at(vma->vm_mm, address, pmdp, pmd);
mm/rmap.c +1 -1
index b30c7c71d1d9..76c8dfd3ae1c 100644
@@ -928,7 +928,7 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
 				continue;
 
 			flush_cache_page(vma, address, page_to_pfn(page));
-			entry = pmdp_huge_clear_flush(vma, address, pmd);
+			entry = pmdp_invalidate(vma, address, pmd);
 			entry = pmd_wrprotect(entry);
 			entry = pmd_mkclean(entry);
 			set_pmd_at(vma->vm_mm, address, pmd, entry);
Signature · 1 line
2.20.1
maintainer
#1 lore raw
Quoted 8 lines · On Thu, 21 Mar 2019 09:36:10 +0530 "Aneesh Kumar K.V"…

MADV_DONTNEED is handled with mmap_sem taken in read mode. We call page_mkclean without holding mmap_sem.

MADV_DONTNEED implies that pages in the region are unmapped and subsequent access to the pages in that range is handled as a new page fault. This implies that if we don't have parallel access to the region when MADV_DONTNEED is run we expect those range to be unallocated.

Quoted 7 lines

w.r.t page_mkclean we need to make sure that we don't break the MADV_DONTNEED semantics. MADV_DONTNEED check for pmd_none without holding pmd_lock. This implies we skip the pmd if we temporarily mark pmd none. Avoid doing that while marking the page clean.

Keep the sequence same for dax too even though we don't support MADV_DONTNEED for dax mapping

What were the runtime effects of the bug?

Did you consider a -stable backport?

author
#2 lore raw
Quoted 15 lines · Andrew Morton
Quoted 13 lines · On Thu, 21 Mar 2019 09:36:10 +0530 "Aneesh Kumar K.V"…

MADV_DONTNEED is handled with mmap_sem taken in read mode. We call page_mkclean without holding mmap_sem.

MADV_DONTNEED implies that pages in the region are unmapped and subsequent access to the pages in that range is handled as a new page fault. This implies that if we don't have parallel access to the region when MADV_DONTNEED is run we expect those range to be unallocated.

w.r.t page_mkclean we need to make sure that we don't break the MADV_DONTNEED semantics. MADV_DONTNEED check for pmd_none without holding pmd_lock. This implies we skip the pmd if we temporarily mark pmd none. Avoid doing that while marking the page clean.

Quoted 4 lines
Quoted 2 lines

Keep the sequence same for dax too even though we don't support MADV_DONTNEED for dax mapping

What were the runtime effects of the bug?

The bug was noticed by code review and I didn't observe any failures w.r.t test run. This is similar to

commit 58ceeb6bec86d9140f9d91d71a710e963523d063

Author: Kirill A. Shutemov <[email protected]>
Date:   Thu Apr 13 14:56:26 2017 -0700

thp: fix MADV_DONTNEED vs. MADV_FREE race

commit ced108037c2aa542b3ed8b7afd1576064ad1362a

Author: Kirill A. Shutemov <[email protected]>
Date:   Thu Apr 13 14:56:20 2017 -0700

thp: fix MADV_DONTNEED vs. numa balancing race

Quoted 2 lines

Did you consider a -stable backport?

Considering nobody reported issues w.r.t MADV_DONTNEED I was not sure.

-aneesh