Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

[David Hildenbrand]

On 24.11.22 10:55, Gavin Shan wrote:
> The issue is reported when removing memory through virtio_mem device.
> The transparent huge page, experienced copy-on-write fault, is wrongly
> regarded as pinned. The transparent huge page is escaped from being
> isolated in isolate_migratepages_block(). The transparent huge page
> can't be migrated and the corresponding memory block can't be put
> into offline state.
>
> Fix it by replacing page_mapcount() with total_mapcount(). With this,
> the transparent huge page can be isolated and migrated, and the memory
> block can be put into offline state. Besides, The page's refcount is
> increased a bit earlier to avoid the page is released when the check
> is executed.

Did you look into handling pages that are in the swapcache case as well?

See is_refcount_suitable() in mm/khugepaged.c.

Should be easy to reproduce, let me know if you need inspiration.

> Fixes: 1da2f328fa64 ("mm,thp,compaction,cma: allow THP migration for CMA allocations")
> Cc: stable@xxxxxxxxxxxxxxx   # v5.7+
> Reported-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> ---
> v2: Corrected fix tag and increase page's refcount before the check
> ---
>   mm/compaction.c | 22 +++++++++++-----------
>   1 file changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c51f7f545afe..1f6da31dd9a5 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -984,29 +984,29 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   			goto isolate_fail;
>   		}
>   
> +		/*
> +		 * Be careful not to clear PageLRU until after we're
> +		 * sure the page is not being freed elsewhere -- the
> +		 * page release code relies on it.
> +		 */
> +		if (unlikely(!get_page_unless_zero(page)))
> +			goto isolate_fail;
> +
>   		/*
>   		 * Migration will fail if an anonymous page is pinned in memory,
>   		 * so avoid taking lru_lock and isolating it unnecessarily in an
>   		 * admittedly racy check.
>   		 */
>   		mapping = page_mapping(page);
> -		if (!mapping && page_count(page) > page_mapcount(page))
> -			goto isolate_fail;
> +		if (!mapping && (page_count(page) - 1) > total_mapcount(page))
> +			goto isolate_fail_put;
>   
>   		/*
>   		 * Only allow to migrate anonymous pages in GFP_NOFS context
>   		 * because those do not depend on fs locks.
>   		 */
>   		if (!(cc->gfp_mask & __GFP_FS) && mapping)
> -			goto isolate_fail;
> -
> -		/*
> -		 * Be careful not to clear PageLRU until after we're
> -		 * sure the page is not being freed elsewhere -- the
> -		 * page release code relies on it.
> -		 */
> -		if (unlikely(!get_page_unless_zero(page)))
> -			goto isolate_fail;
> +			goto isolate_fail_put;
>   
>   		/* Only take pages on LRU: a check now makes later tests safe */
>   		if (!PageLRU(page))

--
Thanks,

David / dhildenb