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

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Message-ID: <CAHc6FU43-n3tk+vvhXKCX+oyUu4x23-vh8pg18wRgYsB0rt+rA@mail.gmail.com>
Date:   Mon, 25 Oct 2021 21:37:22 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Catalin Marinas <catalin.marinas@....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 Fri, Oct 22, 2021 at 8:41 PM Catalin Marinas <catalin.marinas@....com> wrote:
> 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?

It's the EOF case, so it only applies to reads:

        /*
         * We only report that we've read data up to i_size.
         * Revert iter to a state corresponding to that as some callers (such
         * as the splice code) rely on it.
         */
        if (iov_iter_rw(iter) == READ && iomi.pos >= dio->i_size)
                iov_iter_revert(iter, iomi.pos - dio->i_size);

Andreas