Re: [PATCH STABLE v4.19.y] mm/migrate: set swap entry values of THP tail pages properly.

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Mar 06, 2024 at 10:50:52AM -0500, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> The tail pages in a THP can have swap entry information stored in their
> private field. When migrating to a new page, all tail pages of the new
> page need to update ->private to avoid future data corruption.
> 
> This fix is stable-only, since after commit 07e09c483cbe ("mm/huge_memory:
> work on folio->swap instead of page->private when splitting folio"),
> subpages of a swapcached THP no longer requires the maintenance.
> 
> Adding THPs to the swapcache was introduced in commit
> 38d8b4e6bdc87 ("mm, THP, swap: delay splitting THP during swap out"),
> where each subpage of a THP added to the swapcache had its own swapcache
> entry and required the ->private field to point to the correct swapcache
> entry. Later, when THP migration functionality was implemented in commit
> 616b8371539a6 ("mm: thp: enable thp migration in generic path"),
> it initially did not handle the subpages of swapcached THPs, failing to
> update their ->private fields or replace the subpage pointers in the
> swapcache. Subsequently, commit e71769ae5260 ("mm: enable thp migration
> for shmem thp") addressed the swapcache update aspect. This patch fixes
> the update of subpage ->private fields.
> 
> Closes: https://lore.kernel.org/linux-mm/1707814102-22682-1-git-send-email-quic_charante@quicinc.com/
> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/migrate.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 171573613c39..893ea04498f7 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -514,8 +514,12 @@ int migrate_page_move_mapping(struct address_space *mapping,
>  	if (PageSwapBacked(page)) {
>  		__SetPageSwapBacked(newpage);
>  		if (PageSwapCache(page)) {
> +			int i;
> +
>  			SetPageSwapCache(newpage);
> -			set_page_private(newpage, page_private(page));
> +			for (i = 0; i < (1 << compound_order(page)); i++)
> +				set_page_private(newpage + i,
> +						 page_private(page + i));
>  		}
>  	} else {
>  		VM_BUG_ON_PAGE(PageSwapCache(page), page);
> -- 
> 2.43.0
> 
> 

All now queued up, thanks.

greg k-h