linux-kernel - Re: [PATCH v2 3/3] mm: rmap: support batched unmapping for file large folios

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Message-ID: <e718cfdd-a39f-48f6-98c8-f17683f29bcf@lucifer.local>
Date: Tue, 16 Dec 2025 10:54:33 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
        david@...nel.org, catalin.marinas@....com, will@...nel.org,
        ryan.roberts@....com, Liam.Howlett@...cle.com, vbabka@...e.cz,
        rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, riel@...riel.com,
        harry.yoo@...cle.com, jannh@...gle.com, willy@...radead.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] mm: rmap: support batched unmapping for file
 large folios

On Tue, Dec 16, 2025 at 02:22:11PM +0800, Baolin Wang wrote:
>
>
> On 2025/12/16 14:13, Barry Song wrote:
> > > > >
> > > > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > > > index ec232165c47d..4c9d5777c8da 100644
> > > > > --- a/mm/rmap.c
> > > > > +++ b/mm/rmap.c
> > > > > @@ -1855,9 +1855,10 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
> > > > >       end_addr = pmd_addr_end(addr, vma->vm_end);
> > > > >       max_nr = (end_addr - addr) >> PAGE_SHIFT;
> > > > >
> > > > > -    /* We only support lazyfree batching for now ... */
> > > > > -    if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
> > > > > +    /* We only support lazyfree or file folios batching for now ... */
> > > > > +    if (folio_test_anon(folio) && folio_test_swapbacked(folio))
> > > >
> > > > Why is it now ok to support file-backed batched unmapping when it wasn't in
> > > > Barry's series (see [0])? You don't seem to be justifying this?
> > >
> > > Barry's series[0] is merely aimed at optimizing lazyfree anonymous large
> > > folios and does not continue to optimize anonymous large folios or
> > > file-backed large folios at that point.
> >
> > Yep. At that time, I didn’t have an Android machine with a filesystem
> > that supported large folios, so I focused on lazyfree. But I
> > agree that lazyfree anon folios and file folios are quite
> > similar.
> >
> > >
> > > Subsequently, Barry sent out a new patch (see [1]) to optimize anonymous
> > > large folios. As for file-backed large folios, the batched unmapping
> > > support is relatively simple, since we only need to clear the PTE
> > > entries for file-backed large folios.
> >
> > Yep. It is actually quite straightforward to go from lazyfree
> > anon folios to file folios. Swap-backed anon folios are much
> > more tricky, though.
>
> Agree. Thanks Barry for reviewing and confirming.

OK that makes me less concerned, but you do need to put some more justification
in the commit message.

Cheers, Lorenzo