linux-kernel - Re: [PATCH v2] vfs: shave work on failed file open

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Message-ID: <CAHk-=wjUCLfuKks-VGTG9hrFAORb5cuzqyC0gRXptYGGgL=YYg@mail.gmail.com>
Date:   Tue, 26 Sep 2023 12:00:05 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     brauner@...nel.org, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] vfs: shave work on failed file open

On Tue, 26 Sept 2023 at 09:22, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> +void fput_badopen(struct file *file)
> +{
> +       if (unlikely(file->f_mode & (FMODE_BACKING | FMODE_OPENED))) {
> +               fput(file);
> +               return;
> +       }

I don't understand.

Why the FMODE_BACKING test?

The only thing that sets FMODE_BACKING is alloc_empty_backing_file(),
and we know that isn't involved, because the file that is free'd is

        file = alloc_empty_file(op->open_flag, current_cred());

so that test makes no sense.

It might make sense as another WARN_ON_ONCE(), but honestly, why even
that?  Why worry about FMODE_BACKING?

Now, the FMODE_OPENED check makes sense to me, in that it most
definitely can be set, and means we need to call the ->release()
callback and a lot more. Although I get the feeling that this test
would make more sense in the caller, since path_openat() _already_
checks for FMODE_OPENED in the non-error path too.

> +       if (WARN_ON_ONCE(atomic_long_cmpxchg(&file->f_count, 1, 0) != 1)) {
> +               fput(file);
> +               return;
> +       }

Ok, I kind of see why you'd want this safety check.  I don't see how
f_count could be validly anything else, but that's what the
WARN_ON_ONCE is all about.

Anyway, I think I'd be happier about this if it was more of a "just
the reverse of alloc_empty_file()", and path_openat() literally did
just

        if (likely(file->f_mode & FMODE_OPENED))
                release_empty_file(file);
        else
                fput(file);

instead of having this fput_badopen() helper that feels like it needs
to care about other cases than alloc_empty_file().

Don't take this email as a NAK, though. I don't hate the patch. I just
feel it could be more targeted, and more clearly "this is explicitly
avoiding the cost of 'fput()' in just path_openat() if we never
actually filled things in".

                   Linus