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

[Alistair Popple]

Gavin Shan <gshan@xxxxxxxxxx> writes:

> 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.
>
> Fixes: 3917c80280c9 ("thp: change CoW semantics for anon-THP")
> Cc: stable@xxxxxxxxxxxxxxx   # v5.8+
> Reported-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> ---
>  mm/compaction.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c51f7f545afe..c408b5e04c1d 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -990,7 +990,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		 * admittedly racy check.
>  		 */
>  		mapping = page_mapping(page);
> -		if (!mapping && page_count(page) > page_mapcount(page))
> +		if (!mapping && page_count(page) > total_mapcount(page))

We have several versions of these checks for pinned pages open-coded
around the place. See for example migrate_vma_check_page() and
folio_expected_refs(). It looks like you could use a variant of
migrate_vma_check_page() which would also check non-anon pins, although
I don't know the compaction code well enough to know if that's useful.

Either way it would be nice if we had a common helper for these kind of
checks. Guess that would be harder to backport, and the change itself
looks ok. But why isn't the fixes tag 119d6d59dcc0 ("mm, compaction:
avoid isolating pinned pages")?

>  			goto isolate_fail;
>  
>  		/*