linux-kernel - Re: [PATCH] ceph: don't allow copy_file_range when stripe

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Message-ID: <20191031154435.GA30313@hermes.olymp>
Date:   Thu, 31 Oct 2019 15:44:35 +0000
From:   Luis Henriques <lhenriques@...e.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Sage Weil <sage@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
        "Yan, Zheng" <zyan@...hat.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ceph: don't allow copy_file_range when stripe_count != 1

On Thu, Oct 31, 2019 at 11:28:55AM -0400, Jeff Layton wrote:
> On Thu, 2019-10-31 at 11:49 +0000, Luis Henriques wrote:
> > copy_file_range tries to use the OSD 'copy-from' operation, which simply
> > performs a full object copy.  Unfortunately, the implementation of this
> > system call assumes that stripe_count is always set to 1 and doesn't take
> > into account that the data may be striped across an object set.  If the
> > file layout has stripe_count different from 1, then the destination file
> > data will be corrupted.
> > 
> > For example:
> > 
> > Consider a 8 MiB file with 4 MiB object size, stripe_count of 2 and
> > stripe_size of 2 MiB; the first half of the file will be filled with 'A's
> > and the second half will be filled with 'B's:
> > 
> >                0      4M     8M       Obj1     Obj2
> >                +------+------+       +----+   +----+
> >         file:  | AAAA | BBBB |       | AA |   | AA |
> >                +------+------+       |----|   |----|
> >                                      | BB |   | BB |
> >                                      +----+   +----+
> > 
> > If we copy_file_range this file into a new file (which needs to have the
> > same file layout!), then it will start by copying the object starting at
> > file offset 0 (Obj1).  And then it will copy the object starting at file
> > offset 4M -- which is Obj1 again.
> > 
> > Unfortunately, the solution for this is to not allow remote object copies
> > to be performed when the file layout stripe_count is not 1 and simply
> > fallback to the default (VFS) copy_file_range implementation.
> > 
> > Signed-off-by: Luis Henriques <lhenriques@...e.com>
> > ---
> > Hi Jeff,
> > 
> > I hope my understanding of the whole file striping in CephFS is correct;
> > I had to go re-read the whole thing to refresh my memory.
> > 
> > Anyway, I guess that this is not really the only solution to this
> > problem, but it's definitely the simplest one.  copy_file_range is
> > already way more complex that I had ever anticipated.  I would rather
> > keep this simple solution instead of adding more complexity and cover
> > more corner cases.  But yeah, we may want to revisit this in the
> > future...
> > 
> > [OOT: files layout is probably one of the biggest headaches to sort out
> >  the day we want to implement something like FIEMAP on CephFS ;-) ]
> > 
> > Cheers,
> > --
> > Luis
> > 
> >  fs/ceph/file.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> > index d277f71abe0b..3b0e6f9eb6a6 100644
> > --- a/fs/ceph/file.c
> > +++ b/fs/ceph/file.c
> > @@ -1957,9 +1957,12 @@ static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
> >  		return -EOPNOTSUPP;
> >  
> >  	if ((src_ci->i_layout.stripe_unit != dst_ci->i_layout.stripe_unit) ||
> > -	    (src_ci->i_layout.stripe_count != dst_ci->i_layout.stripe_count) ||
> > -	    (src_ci->i_layout.object_size != dst_ci->i_layout.object_size))
> > +	    (src_ci->i_layout.stripe_count != 1) ||
> > +	    (dst_ci->i_layout.stripe_count != 1) ||
> > +	    (src_ci->i_layout.object_size != dst_ci->i_layout.object_size)) {
> > +		dout("Invalid src/dst files layout\n");
> >  		return -EOPNOTSUPP;
> > +	}
> >  
> >  	if (len < src_ci->i_layout.object_size)
> >  		return -EOPNOTSUPP; /* no remote copy will be done */
> 
> I'm fine with restricting CFR to very simple cases, at least initially.
> We can always expand it later once the need becomes clear.
> 
> That said, we should probably add a comment explaining why we're
> excluding cases where the stripe count != 1 here. It doesn't need to
> contain the whole commit log message you wrote, but anyone that does
> want to improve this later might appreciate some breadcrumbs.
> 
> Maybe something like:
> 
> /*
>  * Striped file layouts require that we copy partial objects,
>  * but the OSD copy-from operation only supports full-object copies.
>  * Limit this to non-striped file layouts for now.
>  */
> 
> If that sounds ok, I'll add that in and merge this later today.

Thanks, that looks good to me, feel free to add that comment.

Cheers,
--
Luís