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

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Message-ID: <b8e56515-3903-068c-e4bd-fc0ca5c30d94@google.com>
Date: Mon, 27 Oct 2025 03:49:12 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Kiryl Shutsemau <kirill@...temov.name>
cc: David Hildenbrand <david@...hat.com>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    Matthew Wilcox <willy@...radead.org>, 
    Linus Torvalds <torvalds@...ux-foundation.org>, 
    Alexander Viro <viro@...iv.linux.org.uk>, 
    Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
    Yang Shi <shy828301@...il.com>, Dave Chinner <david@...morbit.com>, 
    Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org, 
    linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/filemap: Implement fast short reads

On Thu, 23 Oct 2025, Yang Shi wrote:
> On Thu, Oct 23, 2025 at 3:34 AM Kiryl Shutsemau <kirill@...temov.name> wrote:
> >
> > On Wed, Oct 22, 2025 at 07:28:27PM +0200, David Hildenbrand wrote:
> > > "garbage" as in pointing at something without a direct map, something that's
> > > protected differently (MTE? weird CoCo protection?) or even worse MMIO with
> > > undesired read-effects.
> >
> > Pedro already points to the problem with missing direct mapping.
> > _nofault() copy should help with this.
> >
> > Can direct mapping ever be converted to MMIO? It can be converted to DMA
> > buffer (which is fine), but MMIO? I have not seen it even in virtualized
> > environments.
> >
> > I cannot say for all CoCo protections, but TDX guest shared<->private
> > should be fine.
> >
> > I am not sure about MTE. Is there a way to bypass MTE check for a load?
> > And how does it deal with stray reads from load_unaligned_zeropad()?
> 
> If I remember correctly, _nofault() copy should skip tag check too.
> 
> Thanks,
> Yang

Not a reply to Yang, I'm just tacking on to the latest mail...

I sew no mention of page migration: __folio_migrate_mapping() does
not use page_cache_delete(), so would surely need to do its own
write_seqcount_begin() and _end(), wouldn't it?

It is using folio_ref_freeze() and _unfreeze(), thinking that
keeps everyone else safely away: but not filemap_read_fast_rcu().

And all other users of folio_ref_freeze() (probably not many but
I have not searched) need to be checked too: maybe no others need
changing, but that has to be established.

This makes a fundamental change to speculative page cache assumptions.

My own opinion was expressed very much better than I could,
by Dave Chinner on Oct 21: cognitive load of niche fastpath.

Hugh