linux-kernel - Re: [PATCH] mm/filemap: Implement fast short reads

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Message-ID: <CAHk-=wjaR_v5Gc_SUGkiz39_hiRHb-AEChknoAu9BUrQRSznAw@mail.gmail.com>
Date: Tue, 21 Oct 2025 18:25:30 -1000
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Dave Chinner <david@...morbit.com>
Cc: Kiryl Shutsemau <kirill@...temov.name>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] mm/filemap: Implement fast short reads

On Tue, 21 Oct 2025 at 13:39, Dave Chinner <david@...morbit.com> wrote:
>
> > > >   1. Locate a folio in XArray.
> > > >   2. Obtain a reference on the folio using folio_try_get().
> > > >   3. If successful, verify that the folio still belongs to
> > > >      the mapping and has not been truncated or reclaimed.
>
> What about if it has been hole-punched?

The sequence number check should take care of anything like that. Do
you have any reason to believe it doesn't?

Yes, you can get the "before or after or between" behavior, but you
can get that with perfectly regular reads that take the refcount on
the page.

Reads have never taken the page lock, and have never been serialized that way.

So the fast case changes absolutely nothing in this respect that I can see.

               Linus