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Subject: Re: [RFC PATCH] ceph: try to prevent exceeding the MDS maximum xattr
 size
To:     =?UTF-8?Q?Lu=c3=ads_Henriques?= <lhenriques@suse.de>,
        Jeff Layton <jlayton@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>
Cc:     ceph-devel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20220520115426.438-1-lhenriques@suse.de>
From:   Xiubo Li <xiubli@redhat.com>
Message-ID: <13988024-efc7-2ab1-036a-eb1d2b2fbd15@redhat.com>
Date:   Mon, 23 May 2022 09:47:41 +0800
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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


> +
>   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;
>