Re: [PATCH v4 0/3] Avoid live-lock in btrfs fault-in+uaccess loop

From: Catalin Marinas <catalin.marinas@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	Josef Bacik <josef@toxicpanda.com>,
	Al Viro <viro@zeniv.linux.org.uk>, Chris Mason <clm@fb.com>,
	David Sterba <dsterba@suse.com>, Will Deacon <will@kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 0/3] Avoid live-lock in btrfs fault-in+uaccess loop
Date: Sat, 23 Apr 2022 19:40:40 +0100	[thread overview]
Message-ID: <YmRIKJSr0xSRHliN@arm.com> (raw)
In-Reply-To: <CAHk-=wgafGgBC9JEu397YxFD8o8qiCZHQS+f5i+BSXOkOFqX3w@mail.gmail.com>

On Sat, Apr 23, 2022 at 09:35:42AM -0700, Linus Torvalds wrote:
> On Sat, Apr 23, 2022 at 3:07 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> >
> > The series introduces fault_in_subpage_writeable() together with the
> > arm64 probing counterpart and the btrfs fix.
> 
> Looks fine to me - and I think it can probably go through the arm64
> tree since you'd be the only one really testing it anyway.

I'll queue it via arm64 then.

> I assume you checked that btrfs is the only one that uses
> fault_in_writeable() in this way? Everybody else updates to the right
> byte boundary and retries (or returns immediately)?

I couldn't find any other places (by inspection or testing). The
buffered file I/O can already make progress in current fault_in_*() +
copy_*_user() loops. O_DIRECT either goes via GUP (and memcpy() doesn't
fault) or, if the user buffer is not PAGE aligned, it may fall back to
buffered I/O. That's why I simplified the series, AFAICT it's only btrfs
search_ioctl() with this problem.

-- 
Catalin