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From:   Nicolas Boichat <drinkcat@chromium.org>
Date:   Fri, 12 Feb 2021 12:48:58 +0800
Message-ID: <CANMq1KCcSZL=_CC2r2Yv8e_p7JY9RKgJ1kSVMrCAO0ZokSupeg@mail.gmail.com>
Subject: Re: [PATCH] fs: generic_copy_file_checks: Do not adjust count based
 on file size
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Dave Chinner <david@fromorbit.com>,
        Luis Lozano <llozano@chromium.org>,
        Ian Lance Taylor <iant@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Amir Goldstein <amir73il@gmail.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Dave Chinner <dchinner@redhat.com>,
        linux-fsdevel@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>
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On Thu, Jan 28, 2021 at 1:57 PM Darrick J. Wong <djwong@kernel.org> wrote:
>
> On Thu, Jan 28, 2021 at 08:46:04AM +0800, Nicolas Boichat wrote:
[snip]
> > Okay, so, based on this and Al's reply, I see 2 things we can do:
> >  1. Go should probably not use copy_file_range in a common library
> > function, as I don't see any easy way to detect this scenario
> > currently (detect 0 size? sure, but that won't work with the example
> > you provide above). And the man page should document this behaviour
> > more explicitly to prevent further incorrect usage.
> >  2. Can procfs/sysfs/debugfs and friends explicitly prevent usage of
> > copy_file_range? (based on Al's reply, there seems to be no way to
> > implement it correctly as seeking in such files will not work in case
> > of short writes)
>
> One /could/ make those three provide a phony CFR implementation that
> would return -EOPNOTSUPP, though like others have said, it's weird to
> have regular files that aren't quite regular.  Not sure where that
> leaves them, though...

Not that simple, as the issue happens on cross-filesystem operations
where file_operations->copy_file_range is not called (and also, that'd
require modifying operations for every single generated file...

Anyway, made an attempt here:
https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/T/#t

> --D
>
> >
> > Thanks,
> >
> > >
> > > Cheers,
> > >
> > > Dave.
> > > --
> > > Dave Chinner
> > > david@fromorbit.com