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From:   Jann Horn <jannh@google.com>
Date:   Sat, 30 Sep 2023 01:02:21 +0200
Message-ID: <CAG48ez3QA7arOtjsUR1FJ_yqyXibK+uftdyrrB3=E0FAYz9g3g@mail.gmail.com>
Subject: Re: [PATCH v2] vfs: shave work on failed file open
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Mateusz Guzik <mjguzik@gmail.com>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
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On Sat, Sep 30, 2023 at 12:24=E2=80=AFAM Matthew Wilcox <willy@infradead.or=
g> wrote:
> On Fri, Sep 29, 2023 at 11:23:04PM +0200, Mateusz Guzik wrote:
> > Extending struct file is not ideal by any means, but the good news is t=
hat:
> > 1. there is a 4 byte hole in there, if one is fine with an int-sized co=
unter
> > 2. if one insists on 8 bytes, the struct is 232 bytes on my kernel
> > (debian). still some room up to 256, so it may be tolerable?
>
> 256 isn't quite the magic number for slabs ... at 256 bytes, we'd get 16
> per 4kB page, but at 232 bytes we get 17 objects per 4kB page (or 35 per
> 8kB pair of pages).
>
> That said, I thik a 32-bit counter is almost certainly sufficient.

I don't like the sequence number proposal because it seems to me like
it's adding one more layer of complication, but if this does happen, I
very much would want that number to be 64-bit. A computer doesn't take
_that_ long to count to 2^32, and especially with preemptible RCU it's
kinda hard to reason about how long a task might stay in the middle of
an RCU grace period. Like, are we absolutely sure that there is no
pessimal case where the scheduler will not schedule a runnable
cpu-pinned idle-priority task for a few minutes? Either because we hit
some pessimal case in the scheduler or because the task gets preempted
by something that's spinning a very long time with preemption
disabled?
(And yes, I know, seqlocks...)