linux-kernel - Re: [PATCH v2 2/3] mm/memory-failure: improve large block size folio handling.

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Message-ID: <03be502e-0979-42cf-a6ba-dea55c4ba375@lucifer.local>
Date: Fri, 17 Oct 2025 10:33:10 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zi Yan <ziy@...dia.com>
Cc: linmiaohe@...wei.com, david@...hat.com, jane.chu@...cle.com,
        kernel@...kajraghav.com,
        syzbot+e6367ea2fdab6ed46056@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com, akpm@...ux-foundation.org,
        mcgrof@...nel.org, nao.horiguchi@...il.com,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Lance Yang <lance.yang@...ux.dev>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Wei Yang <richard.weiyang@...il.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/3] mm/memory-failure: improve large block size folio
 handling.

On Wed, Oct 15, 2025 at 11:34:51PM -0400, Zi Yan wrote:
> Large block size (LBS) folios cannot be split to order-0 folios but
> min_order_for_folio(). Current split fails directly, but that is not
> optimal. Split the folio to min_order_for_folio(), so that, after split,
> only the folio containing the poisoned page becomes unusable instead.
>
> For soft offline, do not split the large folio if it cannot be split to
> order-0. Since the folio is still accessible from userspace and premature
> split might lead to potential performance loss.
>
> Suggested-by: Jane Chu <jane.chu@...cle.com>
> Signed-off-by: Zi Yan <ziy@...dia.com>
> Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>
> ---
>  mm/memory-failure.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index f698df156bf8..443df9581c24 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1656,12 +1656,13 @@ static int identify_page_state(unsigned long pfn, struct page *p,
>   * there is still more to do, hence the page refcount we took earlier
>   * is still needed.
>   */
> -static int try_to_split_thp_page(struct page *page, bool release)
> +static int try_to_split_thp_page(struct page *page, unsigned int new_order,
> +		bool release)
>  {
>  	int ret;
>
>  	lock_page(page);
> -	ret = split_huge_page(page);
> +	ret = split_huge_page_to_list_to_order(page, NULL, new_order);

I wonder if we need a wrapper for these list==NULL cases, as
split_huge_page_to_list_to_order suggests you always have a list provided... and
this is ugly :)

split_huge_page_to_order() seems good.

>  	unlock_page(page);
>
>  	if (ret && release)
> @@ -2280,6 +2281,7 @@ int memory_failure(unsigned long pfn, int flags)
>  	folio_unlock(folio);
>
>  	if (folio_test_large(folio)) {
> +		int new_order = min_order_for_split(folio);

Newline after decl?

>  		/*
>  		 * The flag must be set after the refcount is bumped
>  		 * otherwise it may race with THP split.
> @@ -2294,7 +2296,14 @@ int memory_failure(unsigned long pfn, int flags)
>  		 * page is a valid handlable page.
>  		 */
>  		folio_set_has_hwpoisoned(folio);
> -		if (try_to_split_thp_page(p, false) < 0) {
> +		/*
> +		 * If the folio cannot be split to order-0, kill the process,
> +		 * but split the folio anyway to minimize the amount of unusable
> +		 * pages.
> +		 */
> +		if (try_to_split_thp_page(p, new_order, false) || new_order) {

Please use /* release= */false here


I'm also not sure about the logic here, it feels unclear.

Something like:

	err = try_to_to_split_thp_page(p, new_order, /* release= */false);

		/*
		 * If the folio cannot be split, kill the process.
		 * If it can be split, but not to order-0, then this defeats the
		 * expectation that we do so, but we want the split to have been
		 * made to
		 */

	if (err || new_order > 0) {
	}


> +			/* get folio again in case the original one is split */
> +			folio = page_folio(p);
>  			res = -EHWPOISON;
>  			kill_procs_now(p, pfn, flags, folio);
>  			put_page(p);
> @@ -2621,7 +2630,15 @@ static int soft_offline_in_use_page(struct page *page)
>  	};
>
>  	if (!huge && folio_test_large(folio)) {
> -		if (try_to_split_thp_page(page, true)) {
> +		int new_order = min_order_for_split(folio);
> +
> +		/*
> +		 * If the folio cannot be split to order-0, do not split it at
> +		 * all to retain the still accessible large folio.
> +		 * NOTE: if getting free memory is perferred, split it like it

Typo perferred -> preferred.


> +		 * is done in memory_failure().

I'm confused as to your comment here though, we're not splitting it like
memory_failure()? We're splitting a. with release and b. only if we can target
order-0.

So how would this preference in any way be a thing that happens? :) I may be
missing something here.

> +		 */
> +		if (new_order || try_to_split_thp_page(page, new_order, true)) {

Same comment as above with /* release= */true.

You should pass 0 not new_order to try_to_split_thp_page() here as it has to be
0 for the function to be invoked and that's just obviously clearer.


>  			pr_info("%#lx: thp split failed\n", pfn);
>  			return -EBUSY;
>  		}
> --
> 2.51.0
>