Re: [RFC PATCH] mm/rmap.c: finer hwpoison granularity for PTE-mapped THP

["Kirill A. Shutemov" <kirill@shutemov.name>]

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