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SubjectRe: [RFC PATCH] mm/rmap.c: finer hwpoison granularity for PTE-mapped THP
On Thu, Jan 02, 2020 at 11:04:21AM +0800, Wei Yang wrote:
> Currently we behave differently between PMD-mapped THP and PTE-mapped
> THP on memory_failure.
>
> User detected difference:
>
> For PTE-mapped THP, the whole 2M range will trigger MCE after
> memory_failure(), while only 4K range for PMD-mapped THP will.
>
> Direct reason:
>
> All the 512 PTE entry will be marked as hwpoison entry for a PTE-mapped
> THP while only one PTE will be marked for a PMD-mapped THP.
>
> Root reason:
>
> The root cause is PTE-mapped page doesn't need to split pmd which skip
> the SPLIT_FREEZE process.

I don't follow how SPLIT_FREEZE is related to pisoning. Cold you
laraborate?

> This makes try_to_unmap_one() do its job when
> the THP is not splited. And since page is HWPOISON, all the entries in
> THP is marked as hwpoison entry.
>
> While for the PMD-mapped THP, SPLIT_FREEZE will save migration entry to
> pte and this skip try_to_unmap_one() before THP splited. And then only
> the affected 4k page is marked as hwpoison entry.
>
> This patch tries to provide a finer granularity for PTE-mapped THP by
> only mark the affected subpage as hwpoison entry when THP is not
> split.
>
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>
> ---
> This complicates the picture a little, while I don't find a better way to
> improve.
>
> Also I may miss some case or not handle this properly.
>
> Look forward your comments.
> ---
> mm/rmap.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index b3e381919835..90229917dd64 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1554,10 +1554,11 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> set_huge_swap_pte_at(mm, address,
> pvmw.pte, pteval,
> vma_mmu_pagesize(vma));
> - } else {
> + } else if (!PageAnon(page) || page == subpage) {
> dec_mm_counter(mm, mm_counter(page));
> set_pte_at(mm, address, pvmw.pte, pteval);
> - }
> + } else
> + goto freeze;
>
> } else if (pte_unused(pteval) && !userfaultfd_armed(vma)) {
> /*
> @@ -1579,6 +1580,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> swp_entry_t entry;
> pte_t swp_pte;
>
> +freeze:
> if (arch_unmap_one(mm, vma, address, pteval) < 0) {
> set_pte_at(mm, address, pvmw.pte, pteval);
> ret = false;
> --
> 2.17.1
>
>

--
Kirill A. Shutemov

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