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thp: fix MADV_DONTNEED vs. numa balancing race

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In case prot_numa, we are under down_read(mmap_sem).  It's critical to
not clear pmd intermittently to avoid race with MADV_DONTNEED which is
also under down_read(mmap_sem):

	CPU0:				CPU1:
				change_huge_pmd(prot_numa=1)
				 pmdp_huge_get_and_clear_notify()
madvise_dontneed()
 zap_pmd_range()
  pmd_trans_huge(*pmd) == 0 (without ptl)
  // skip the pmd
				 set_pmd_at();
				 // pmd is re-established

The race makes MADV_DONTNEED miss the huge pmd and don't clear it
which may break userspace.

Found by code analysis, never saw triggered.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Kirill A. Shutemov authored and Linus Torvalds committed 2017-04-13 18:24:20 -07:00
1 parent 0a85e51d37
commit ced108037c
1 file changed
+33 -1
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mm/huge_memory.c
+33 -1
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@@ -1746,7 +1746,39 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
if (prot_numa && pmd_protnone(*pmd))
goto unlock;
entry = pmdp_huge_get_and_clear_notify(mm, addr, pmd);
/*
* In case prot_numa, we are under down_read(mmap_sem). It's critical
* to not clear pmd intermittently to avoid race with MADV_DONTNEED
* which is also under down_read(mmap_sem):
*
* CPU0: CPU1:
* change_huge_pmd(prot_numa=1)
* pmdp_huge_get_and_clear_notify()
* madvise_dontneed()
* zap_pmd_range()
* pmd_trans_huge(*pmd) == 0 (without ptl)
* // skip the pmd
* set_pmd_at();
* // pmd is re-established
*
* The race makes MADV_DONTNEED miss the huge pmd and don't clear it
* which may break userspace.
*
* pmdp_invalidate() is required to make sure we don't miss
* dirty/young flags set by hardware.
*/
entry = *pmd;
pmdp_invalidate(vma, addr, pmd);
/*
* Recover dirty/young flags. It relies on pmdp_invalidate to not
* corrupt them.
*/
if (pmd_dirty(*pmd))
entry = pmd_mkdirty(entry);
if (pmd_young(*pmd))
entry = pmd_mkyoung(entry);
entry = pmd_modify(entry, newprot);
if (preserve_write)
entry = pmd_mk_savedwrite(entry);