linux-kernel - Re: [PATCH 1/2] mm: vmscan: remove folio_test

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Message-ID: <aMh7SAlitp5FqR-M@casper.infradead.org>
Date: Mon, 15 Sep 2025 21:47:04 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, david@...hat.com,
	mhocko@...nel.org, zhengqi.arch@...edance.com,
	shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
	hughd@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: vmscan: remove folio_test_private() check in
 pageout()

On Sat, Sep 13, 2025 at 11:04:48AM +0800, Baolin Wang wrote:
> On 2025/9/12 23:21, Matthew Wilcox wrote:
> > On Fri, Sep 12, 2025 at 11:45:07AM +0800, Baolin Wang wrote:
> > > @@ -697,22 +697,8 @@ static pageout_t pageout(struct folio *folio, struct address_space *mapping,
> > >   	 * swap_backing_dev_info is bust: it doesn't reflect the
> > >   	 * congestion state of the swapdevs.  Easy to fix, if needed.
> > >   	 */
> > > -	if (!is_page_cache_freeable(folio))
> > > +	if (!is_page_cache_freeable(folio) || !mapping)
> > >   		return PAGE_KEEP;
> > 
> > I feel like we need to keep the comment (assuming it's still true ...
> > which it probably is, although there's nobody who would think to update
> > this comment if it became no longer true).  I would certainly wonder why
> > we can have this !mapping test.
> 
> I think the !mapping check is still needed here because the tmpfs/shmem
> folios truncation might race with folio reclamation, see shmem_undo_range().

I agree that we still need the !mapping check.  But it needs this comment
that you're deleting, because it's not obvious why we'd have a dirty
folio with a NULL mapping on the LRU list.

> > > -		/*
> > > -		 * Some data journaling orphaned folios can have
> > > -		 * folio->mapping == NULL while being dirty with clean buffers.
> > > -		 */