Date:   Thu, 25 Nov 2021 11:10:43 +0000
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        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>
Subject: Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware
 with sub-page faults
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On Wed, Nov 24, 2021 at 03:00:00PM -0800, Linus Torvalds wrote:
> 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".
> 
> Looking at 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.

I agree, if we want the number of byte not copied, we should just change
copy_{to,from}_user_nofault() and their callers (I can count three
each).

For this specific btrfs case, if we want go with tuning the offset based
on the fault address, we'd need copy_to_user_nofault() (or a new
function) to be exact. IOW, fall back to byte-at-a-time copy until it
hits the real faulting address.

-- 
Catalin