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

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Message-ID: <YmRIKJSr0xSRHliN@arm.com>
Date:   Sat, 23 Apr 2022 19:40:40 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Josef Bacik <josef@...icpanda.com>,
        Al Viro <viro@...iv.linux.org.uk>, Chris Mason <clm@...com>,
        David Sterba <dsterba@...e.com>, Will Deacon <will@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/3] Avoid live-lock in btrfs fault-in+uaccess loop

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@....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