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

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Message-ID: <874k1gmvhh.fsf@brahms.olymp>
Date:   Mon, 23 May 2022 10:47:06 +0100
From:   Luís Henriques <lhenriques@...e.de>
To:     Xiubo Li <xiubli@...hat.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ceph: try to prevent exceeding the MDS maximum
 xattr size

Xiubo Li <xiubli@...hat.com> writes:

> 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].
>

Right, as I mentioned in that PR, having clients that are aware of that
limit will allow them to behave correctly.  But the MDS still needs to
handle the case where these limits are exceeded (for ex. kernel clients
that don't know about this limit).  So, we still need to do something
similar to what I've proposed in there.

Cheers
-- 
Luís

> @Jeff, any better idea ?
>
>
> [1]
> https://github.com/ceph/ceph/pull/46357/commits/741f8ba36f14774834c0d5618519425ccf1ccc85#r878966753
>
> Thanks.
>
> -- Xiubo
>
>
>> +
>>   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;
>>
>