Re: [PATCH] mm: thp: check total_mapcount instead of page_mapcount

["Zi Yan" <ziy@nvidia.com>]

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On 30 Apr 2021, at 17:07, Yang Shi wrote:

> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> return false positive for PTE-mapped THP since page_mapcount() is used
> to check if the THP is unmapped, but it just checks compound mapount and
> head page's mapcount.  If the THP is PTE-mapped and head page is not
> mapped, it may return false positive.
>
> Use total_mapcount() instead of page_mapcount() and do so for the
> VM_BUG_ON_PAGE in split_huge_page_to_list as well.
>
> [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/
>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  mm/huge_memory.c | 2 +-
>  mm/rmap.c        | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 63ed6b25deaa..2122c3e853b9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	}
>
>  	unmap_page(head);
> -	VM_BUG_ON_PAGE(compound_mapcount(head), head);
> +	VM_BUG_ON_PAGE(total_mapcount(head), head);

I am not sure about this change. The code below also checks total_mapcount(head)
and returns EBUSY if the count is non-zero. This change makes the code dead.
On the other hand, the change will force all mappings to the page have to be
successfully unmapped all the time. I am not sure if we want to do that.
Maybe it is better to just check total_mapcount() and fail the split.
The same situation happens with the code change below.

>
>  	/* block interrupt reentry in xa_lock and spinlock */
>  	local_irq_disable();
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 693a610e181d..2e547378ab5f 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
>  	else
>  		rmap_walk(page, &rwc);
>
> -	return !page_mapcount(page) ? true : false;
> +	return !total_mapcount(page) ? true : false;
>  }

In unmap_page(), VM_BUG_ON_PAGE(!unmap_success, page) will force all mappings
to the page have to be all unmapped, which might not be the case we want.
Maybe you will want to remove the VM_BUG_ON_PAGE here, check total_mapcount()
above, and fail the split if not all mappings to the pages are unmapped.



—
Best Regards,
Yan Zi

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