linux-kernel - Re: [RFC PATCH] ceph: try to prevent exceeding the MDS maximum xattr size

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Date:   Mon, 23 May 2022 06:43:51 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Xiubo Li <xiubli@...hat.com>,
        Luís Henriques <lhenriques@...e.de>,
        Ilya Dryomov <idryomov@...il.com>
Cc:     ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ceph: try to prevent exceeding the MDS maximum
 xattr size

On Mon, 2022-05-23 at 09:47 +0800, Xiubo Li wrote:
> On 5/20/22 7:54 PM, Luís Henriques wrote:
> > The MDS tries to enforce a limit on the total key/values in extended
> > attributes.  However, this limit is enforced only if doing a synchronous
> > operation (MDS_OP_SETXATTR) -- if we're buffering the xattrs, the MDS
> > doesn't have a chance to enforce these limits.
> > 
> > This patch forces the usage of the synchronous operation if xattrs size hits
> > the maximum size that is set on the MDS by default (64k).
> > 
> > While there, fix a dout() that would trigger a printk warning:
> > 
> > [   98.718078] ------------[ cut here ]------------
> > [   98.719012] precision 65536 too large
> > [   98.719039] WARNING: CPU: 1 PID: 3755 at lib/vsprintf.c:2703 vsnprintf+0x5e3/0x600
> > ...
> > 
> > URL: https://tracker.ceph.com/issues/55725
> > Signed-off-by: Luís Henriques <lhenriques@...e.de>
> > ---
> >   fs/ceph/xattr.c | 17 +++++++++++++----
> >   1 file changed, 13 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/ceph/xattr.c b/fs/ceph/xattr.c
> > index afec84088471..09751a5f028c 100644
> > --- a/fs/ceph/xattr.c
> > +++ b/fs/ceph/xattr.c
> > @@ -15,6 +15,12 @@
> >   #define XATTR_CEPH_PREFIX "ceph."
> >   #define XATTR_CEPH_PREFIX_LEN (sizeof (XATTR_CEPH_PREFIX) - 1)
> >   
> > +/*
> > + * Maximum size of xattrs the MDS can handle per inode by default.  This
> > + * includes the attribute name and 4+4 bytes for the key/value sizes.
> > + */
> > +#define MDS_MAX_XATTR_PAIRS_SIZE (1<<16) /* 64K */
> 
> The max size is changeable in MDS side. Possibly we should do something 
> as mentioned in your ceph PR [1].
> 
> @Jeff, any better idea ?
> 
> 
> [1] 
> https://github.com/ceph/ceph/pull/46357/commits/741f8ba36f14774834c0d5618519425ccf1ccc85#r878966753
> 
> Thanks.
> 
> -- Xiubo
> 
> 

Not really.

The idea in the github comment makes sense. Basically, make it so that
the setting isn't changeable at runtime and then have the client query
for the limit at appropriate times.

You can probably still defeat that by changing it and rebooting the MDS,
but I don't see that you can do much else.

> > +
> >   static int __remove_xattr(struct ceph_inode_info *ci,
> >   			  struct ceph_inode_xattr *xattr);
> >   
> > @@ -1078,7 +1084,7 @@ static int ceph_sync_setxattr(struct inode *inode, const char *name,
> >   			flags |= CEPH_XATTR_REMOVE;
> >   	}
> >   
> > -	dout("setxattr value=%.*s\n", (int)size, value);
> > +	dout("setxattr value size: ld\n", size);
> >   
> >   	/* do request */
> >   	req = ceph_mdsc_create_request(mdsc, op, USE_AUTH_MDS);
> > @@ -1176,8 +1182,13 @@ int __ceph_setxattr(struct inode *inode, const char *name,
> >   	spin_lock(&ci->i_ceph_lock);
> >   retry:
> >   	issued = __ceph_caps_issued(ci, NULL);
> > -	if (ci->i_xattrs.version == 0 || !(issued & CEPH_CAP_XATTR_EXCL))
> > +	required_blob_size = __get_required_blob_size(ci, name_len, val_len);
> > +	if ((ci->i_xattrs.version == 0) || !(issued & CEPH_CAP_XATTR_EXCL) ||
> > +	    (required_blob_size >= MDS_MAX_XATTR_PAIRS_SIZE)) {
> > +		dout("%s do sync setxattr: version: %llu blob size: %d\n",
> > +		     __func__, ci->i_xattrs.version, required_blob_size);
> >   		goto do_sync;
> > +	}
> >   
> >   	if (!lock_snap_rwsem && !ci->i_head_snapc) {
> >   		lock_snap_rwsem = true;
> > @@ -1193,8 +1204,6 @@ int __ceph_setxattr(struct inode *inode, const char *name,
> >   	     ceph_cap_string(issued));
> >   	__build_xattrs(inode);
> >   
> > -	required_blob_size = __get_required_blob_size(ci, name_len, val_len);
> > -
> >   	if (!ci->i_xattrs.prealloc_blob ||
> >   	    required_blob_size > ci->i_xattrs.prealloc_blob->alloc_len) {
> >   		struct ceph_buffer *blob;
> > 
> 

-- 
Jeff Layton <jlayton@...nel.org>