linux-kernel - Re: [RFC PATCH] ceph: fix cross quota realms renames with new truncated files

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Date:   Wed, 11 Nov 2020 14:33:06 -0500
From:   Jeff Layton <jlayton@...nel.org>
To:     Luis Henriques <lhenriques@...e.de>
Cc:     Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ceph: fix cross quota realms renames with new
 truncated files

On Wed, 2020-11-11 at 18:28 +0000, Luis Henriques wrote:
> Jeff Layton <jlayton@...nel.org> writes:
> 
> > On Wed, 2020-11-11 at 15:39 +0000, Luis Henriques wrote:
> > > When doing a rename across quota realms, there's a corner case that isn't
> > > handled correctly.  Here's a testcase:
> > > 
> > >   mkdir files limit
> > >   truncate files/file -s 10G
> > >   setfattr limit -n ceph.quota.max_bytes -v 1000000
> > >   mv files limit/
> > > 
> > > The above will succeed because ftruncate(2) won't result in an immediate
> > > notification of the MDSs with the new file size, and thus the quota realms
> > > stats won't be updated.
> > > 
> > > This patch forces a sync with the MDS every time there's an ATTR_SIZE that
> > > sets a new i_size, even if we have Fx caps.
> > > 
> > > Cc: stable@...r.kernel.org
> > > Fixes: dffdcd71458e ("ceph: allow rename operation under different quota realms")
> > > URL: https://tracker.ceph.com/issues/36593
> > > Signed-off-by: Luis Henriques <lhenriques@...e.de>
> > > ---
> > >  fs/ceph/inode.c | 11 ++---------
> > >  1 file changed, 2 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
> > > index 526faf4778ce..30e3f240ac96 100644
> > > --- a/fs/ceph/inode.c
> > > +++ b/fs/ceph/inode.c
> > > @@ -2136,15 +2136,8 @@ int __ceph_setattr(struct inode *inode, struct iattr *attr)
> > >  	if (ia_valid & ATTR_SIZE) {
> > >  		dout("setattr %p size %lld -> %lld\n", inode,
> > >  		     inode->i_size, attr->ia_size);
> > > -		if ((issued & CEPH_CAP_FILE_EXCL) &&
> > > -		    attr->ia_size > inode->i_size) {
> > > -			i_size_write(inode, attr->ia_size);
> > > -			inode->i_blocks = calc_inode_blocks(attr->ia_size);
> > > -			ci->i_reported_size = attr->ia_size;
> > > -			dirtied |= CEPH_CAP_FILE_EXCL;
> > > -			ia_valid |= ATTR_MTIME;
> > > -		} else if ((issued & CEPH_CAP_FILE_SHARED) == 0 ||
> > > -			   attr->ia_size != inode->i_size) {
> > > +		if ((issued & (CEPH_CAP_FILE_EXCL|CEPH_CAP_FILE_SHARED)) ||
> > > +		    (attr->ia_size != inode->i_size)) {
> > >  			req->r_args.setattr.size = cpu_to_le64(attr->ia_size);
> > >  			req->r_args.setattr.old_size =
> > >  				cpu_to_le64(inode->i_size);
> > 
> > Hmm...this makes truncates more expensive when we have caps. I'd rather
> > not do that if we can help it.
> 
> Yeah, as I mentioned in the tracker, there's indeed a performance impact
> with this fix.  That's what made me add the RFC in the subject ;-)
> 
> > What about instead having the client mimic a fsync when there is a
> > rename across quota realms? If we can't tell that reliably then we could
> > also just do an effective fsync ahead of any cross-directory rename?
> 
> Ok, thanks for the suggestion.  That may actually work, although it will
> make the rename more expensive of course.  I'll test that tomorrow and
> eventually follow-up with a patch.

In principle, there should only be an impact when the file being renamed
has dirty data and is crossing quota realms. I'd much rather slow down
the rename than truncate in this case. open(..., O_TRUNC) is _very_
common.
-- 
Jeff Layton <jlayton@...nel.org>