linux-kernel - Re: [PATCH] pipe: cache 2 pages instead of 1

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Message-ID: <CAGudoHG7EF5_wnNhsyFoiRtU-qW1b=vUaVaFk7TKnqeSjC6sOg@mail.gmail.com>
Date: Thu, 27 Feb 2025 23:07:45 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] pipe: cache 2 pages instead of 1

On Thu, Feb 27, 2025 at 10:59 PM Oleg Nesterov <oleg@...hat.com> wrote:
> > +static struct page *anon_pipe_get_page(struct pipe_inode_info *pipe)
> > +{
> > +     struct page *page;
> > +
> > +     if (pipe->tmp_page[0]) {
> > +             page = pipe->tmp_page[0];
> > +             pipe->tmp_page[0] = NULL;
> > +     } else if (pipe->tmp_page[1]) {
> > +             page = pipe->tmp_page[1];
> > +             pipe->tmp_page[1] = NULL;
> > +     } else {
> > +             page = alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT);
> > +     }
> > +
> > +     return page;
> > +}
>
> Perhaps something like
>
>         for (i = 0; i < ARRAY_SIZE(pipe->tmp_page); i++) {
>                 if (pipe->tmp_page[i]) {
>                         struct page *page = pipe->tmp_page[i];
>                         pipe->tmp_page[i] = NULL;
>                         return page;
>                 }
>         }
>
>         return alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT);
> ?
>
> Same for anon_pipe_put_page() and free_pipe_info().
>
> This avoids the code duplication and allows to change the size of
> pipe->tmp_page[] array without other changes.
>

I have almost no opinion one way or the other and I'm not going to
argue about this bit. I only note I don't expect there is a legit
reason to go beyond 2 pages here. As in if more is warranted, the
approach to baking the area should probably change.

I started with this being spelled out so that I have easier time
toggling the extra slot for testing.

That said, I don't know who counts as the pipe man today. I can do the
needful(tm) no problem.
-- 
Mateusz Guzik <mjguzik gmail.com>