Re: Optimizing small reads

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, 3 Oct 2025 at 09:18, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> That was one of the theories and it might be the right one. There was
> some discussion about having a sequence number. It's a long time ago,
> I forget.

Ok, this bothered me. So I went back.

Damn. I actually mentioned that in the original submission (well,
actually the reply after I had booted into it [1]):

 "And even then there's the question about replacing the same folio in
  the same spot in the xarray. I'm not convinced it is worth worrying
  about in any reality we care about, but it's _technically_ all a bit
  wrong"

but then I left it because I was hoping somebody else would deal with it.

And I brought up the patch again several months later, but by then I
had obviously repressed that issue.

So yeah, I think that's it, and I even knew about it originally, but
in my incompetence I had entirely forgotten.

And by "that's it", I obviously mean that there might be something
else going on too.

How about a sequence number in 'struct address_space' that gets
incremented by removing folios?

Or even in 'struct xarray': it would always be protected by xa_lock,
so it doesn't even need any new atomicity...

Anyway, I think you're entirely right, Kirill. And sorry for wasting
peoples time when I _should_ have remembered about this.

        Linus

https://lore.kernel.org/all/CAHk-=whFzKO+Vx2f8h72m7ysiEP2Thubp3EaZPa7kuoJb0k=ig@mail.gmail.com/
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