linux-kernel - Re: [RFC][arm64] possible infinite loop in btrfs search

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Message-ID: <YXMFw34ZpW+CwlmI@arm.com>
Date:   Fri, 22 Oct 2021 19:41:07 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        Josef Bacik <josef@...icpanda.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()

On Thu, Oct 21, 2021 at 08:00:50PM +0200, Andreas Gruenbacher wrote:
> On Thu, Oct 21, 2021 at 7:09 PM Catalin Marinas <catalin.marinas@....com> wrote:
> > This discussion started with the btrfs search_ioctl() where, even if
> > some bytes were written in copy_to_sk(), it always restarts from an
> > earlier position, reattempting to write the same bytes. Since
> > copy_to_sk() doesn't guarantee forward progress even if some bytes are
> > writable, Linus' suggestion was for fault_in_writable() to probe the
> > whole range. I consider this overkill since btrfs is the only one that
> > needs probing every 16 bytes. The other cases like the new
> > fault_in_safe_writeable() can be fixed by probing the first byte only
> > followed by gup.
> 
> Hmm. Direct I/O request sizes are multiples of the underlying device
> block size, so we'll also get stuck there if fault-in won't give us a
> full block. This is getting pretty ugly. So scratch that idea; let's
> stick with probing the whole range.

Ah, I wasn't aware of this. I got lost in the call trees but I noticed
__iomap_dio_rw() does an iov_iter_revert() only if direction is READ. Is
this the case for writes as well?

-- 
Catalin