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

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Message-ID: <87ee0o2sir.fsf@brahms.olymp>
Date:   Fri, 20 May 2022 15:22:20 +0100
From:   Luís Henriques <lhenriques@...e.de>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Xiubo Li <xiubli@...hat.com>, 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

Luís Henriques <lhenriques@...e.de> writes:

> 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 */
> +
>  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);

Oops!  Looks like someone ate a '%' char.  Oh well, I won't bother sending
out a new version for now as this is an RFC and the MDS side is what
really needs fixing.  In fact, the client-side may be something very
different from this RFC.

Cheers,
-- 
Luís

>  
>  	/* 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;