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

[Xiubo Li <xiubli@redhat.com>]

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@suse.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.

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;
>>>