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

[Xiubo Li]

On 5/23/22 6:43 PM, Jeff Layton wrote:
> 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@xxxxxxx>
> > > ---
> > >    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.

Yeah, sounds good.


> > > +
> > >    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;