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SubjectRe: [PATCH 2/4] thp: fix MADV_DONTNEED vs. numa balancing race
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On 03/02/2017 04:10 PM, Kirill A. Shutemov wrote:
> 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.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/huge_memory.c | 34 +++++++++++++++++++++++++++++++++-
> 1 file changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e7ce73b2b208..bb2b3646bd78 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1744,7 +1744,39 @@ 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.
> + */

pmdp_invalidate() does:

pmd_t entry = *pmdp;
set_pmd_at(vma->vm_mm, address, pmdp, pmd_mknotpresent(entry));

so it's not atomic and if CPU sets dirty or accessed in the middle of
this, they will be lost?

But I don't see how the other invalidate caller
__split_huge_pmd_locked() deals with this either. Andrea, any idea?

Vlastimil

> + 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);
>

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