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 <20211124192024.2408218-4-catalin.marinas@arm.com> <YZ6arlsi2L3LVbFO@casper.infradead.org>
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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 24 Nov 2021 15:00:00 -0800
Message-ID: <CAHk-=wgHqjX3kenSk5_bCRM+ZC-tgndBMfbVVsbp0CwJf2DU-w@mail.gmail.com>
Subject: Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware
 with sub-page faults
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>
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Catalin talked about the other change, but this part:

On Wed, Nov 24, 2021 at 12:04 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> (where __copy_to_user_nofault() is a new function that does exactly what
> copy_to_user_nofault() does, but returns the number of bytes copied)

If we want the "how many bytes" part, then we should just make
copy_to_user_nofault() have the same semantics as a plain
copy_to_user().

IOW, change it to return "number of bytes not copied".

Lookin gat the current uses, such a change would be trivial. The only
case that wants a 0/-EFAULT error is the bpf_probe_write_user(),
everybody else already just wants "zero for success", so changing
copy_to_user_nofault() would be trivial.

And it really is odd and very non-intuitive that
copy_to_user_nofault() has a completely different return value from
copy_to_user().

So if _anybody_ wants a byte-count, that should just be fixed.

                    Linus