linux-kernel - Re: [PATCH 04/12] mm/vmscan: Free non-shmem folios without splitting them

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Message-ID: <20220117160625.oofpzl7tqm5udwaj@box.shutemov.name>
Date:   Mon, 17 Jan 2022 19:06:25 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 04/12] mm/vmscan: Free non-shmem folios without splitting
 them

On Sun, Jan 16, 2022 at 12:18:14PM +0000, Matthew Wilcox (Oracle) wrote:
> We have to allocate memory in order to split a file-backed folio, so
> it's not a good idea to split them in the memory freeing path.

Could elaborate on why split a file-backed folio requires memory
allocation?

> It also
> doesn't work for XFS because pages have an extra reference count from
> page_has_private() and split_huge_page() expects that reference to have
> already been removed.

Need to adjust can_split_huge_page()?

> Unfortunately, we still have to split shmem THPs
> because we can't handle swapping out an entire THP yet.

... especially if the system doesn't have swap :P

> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  mm/vmscan.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 700434db5735..45665874082d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1728,8 +1728,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  				/* Adding to swap updated mapping */
>  				mapping = page_mapping(page);
>  			}
> -		} else if (unlikely(PageTransHuge(page))) {
> -			/* Split file THP */
> +		} else if (PageSwapBacked(page) && PageTransHuge(page)) {
> +			/* Split shmem THP */
>  			if (split_huge_page_to_list(page, page_list))
>  				goto keep_locked;
>  		}
> -- 
> 2.34.1
> 

-- 
 Kirill A. Shutemov