linux-kernel - Re: [PATCH 01/39] vfs: dedpue: return loff

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Date:   Wed, 6 Jun 2018 08:09:05 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-xfs <linux-xfs@...r.kernel.org>, ocfs2-devel@....oracle.com
Subject: Re: [PATCH 01/39] vfs: dedpue: return loff_t

On Tue, Jun 05, 2018 at 10:33:22AM +0200, Miklos Szeredi wrote:
> On Mon, Jun 4, 2018 at 10:43 AM, Christoph Hellwig <hch@...radead.org> wrote:
> > On Tue, May 29, 2018 at 04:43:01PM +0200, Miklos Szeredi wrote:
> >> f_op->dedupe_file_range() gets a u64 length to dedup and returns an ssize_t
> >> actual length deduped.  This breaks badly on 32bit archs since the returned
> >> length will be truncated and possibly overflow into the sign bit (xfs and
> >> ocfs2 are affected, btrfs limits actual length to 16MiB).
> >
> > Can we just make it return 0 vs errno?  The only time we return
> > a different length than the one passed in is due to the btrfs cap.
> >
> > Given that this API started out on btrfs we should just do the cap
> > everywhere to not confuse userspace.
> 
> And that's a completely arbitrary cap; sure btrfs started out with
> that, but there's no fundamental reason for that becoming the global
> limit.  Xfs now added a different, larger limit, so based on what
> reason should that limit be reduced?
> 
> I don't care either way, but at this stage I'm not going to change
> this patch, unless there's a very good reason to do so, because if I
> do someone will come and suggest another improvement, ad-infinitum...

I think we should hoist the MAX_RW_COUNT/2 limit to the VFS helpers
since afaict we generally cap max IO per call at MAX_RW_COUNT.  (I
probably should've capped ocfs2 back when I did xfs, but forgot).  If
btrfs wants to keep their lower (16M) limit then they're free to do so;
the interface documentation allows for this.  One of the btrfs
developers seems to be working on a patch series to raise the limit[1]
anyway.

--D

[1] https://www.spinics.net/lists/linux-btrfs/msg78392.html

> 
> Thanks,
> Miklos