Re: any idea about auto export multiple btrfs snapshots?

[Wang Yugui <wangyugui@e16-tech.com>]

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Hi,

> On Wed, 23 Jun 2021, Wang Yugui wrote:
> > Hi,
> > 
> > This patch works very well. Thanks a lot.
> > -  crossmnt of btrfs subvol works as expected.
> > -  nfs/umount subvol works well.
> > -  pseudo mount point inode(255) is good.
> > 
> > I test it in 5.10.45 with a few minor rebase.
> > ( see 0001-any-idea-about-auto-export-multiple-btrfs-snapshots.patch,
> > just fs/nfsd/nfs3xdr.c rebase)
> > 
> > But when I tested it with another btrfs system without subvol but with
> > more data, 'find /nfs/test' caused a OOPS .  and this OOPS will not
> > happen just without this patch.
> > 
> > The data in this filesystem is created/left by xfstest(FSTYP=nfs,
> > TEST_DEV).
> > 
> > #nfs4 option: default mount.nfs4, nfs-utils-2.3.3
> > 
> > # access btrfs directly
> > $ find /mnt/test | wc -l
> > 6612
> > 
> > # access btrfs through nfs
> > $ find /nfs/test | wc -l
> > 
> > [  466.164329] BUG: kernel NULL pointer dereference, address: 0000000000000004
> > [  466.172123] #PF: supervisor read access in kernel mode
> > [  466.177857] #PF: error_code(0x0000) - not-present page
> > [  466.183601] PGD 0 P4D 0
> > [  466.186443] Oops: 0000 [#1] SMP NOPTI
> > [  466.190536] CPU: 27 PID: 1819 Comm: nfsd Not tainted 5.10.45-7.el7.x86_64 #1
> > [  466.198418] Hardware name: Dell Inc. PowerEdge T620/02CD1V, BIOS 2.9.0 12/06/2019
> > [  466.206806] RIP: 0010:fsid_source+0x7/0x50 [nfsd]
> 
> in nfsd4_encode_fattr there is code:
> 
> 	if ((bmval0 & (FATTR4_WORD0_FILEHANDLE | FATTR4_WORD0_FSID)) && !fhp) {
> 		tempfh = kmalloc(sizeof(struct svc_fh), GFP_KERNEL);
> 		status = nfserr_jukebox;
> 		if (!tempfh)
> 			goto out;
> 		fh_init(tempfh, NFS4_FHSIZE);
> 		status = fh_compose(tempfh, exp, dentry, NULL);
> 		if (status)
> 			goto out;
> 		fhp = tempfh;
> 	}
> 
> Change that to test for (bmval1 & FATTR4_WORD1_MOUNTED_ON_FILEID) as
> well.
> 
> NeilBrown


It works well.

-	if ((bmval0 & (FATTR4_WORD0_FILEHANDLE | FATTR4_WORD0_FSID)) && !fhp) {
+	if (((bmval0 & (FATTR4_WORD0_FILEHANDLE | FATTR4_WORD0_FSID)) ||
+		 (bmval1 & FATTR4_WORD1_MOUNTED_ON_FILEID))
+		 && !fhp) {
 		tempfh = kmalloc(sizeof(struct svc_fh), GFP_KERNEL);
 		status = nfserr_jukebox;
 		if (!tempfh)


And I tested some case about statfs.f_fsid conflict between btrfs
filesystem, and it works well too. 

Is it safe in theory too?

test case:
two btrfs filesystem with just 1 bit diff of UUID
# blkid /dev/sdb1 /dev/sdb2
/dev/sdb1: UUID="35327ecf-a5a7-4617-a160-1fdbfd644940" UUID_SUB="a831ebde-1e66-4592-bfde-7a86fd6478b5" BLOCK_SIZE="4096" TYPE="btrfs" PARTLABEL="primary" PARTUUID="3e30a849-88db-4fb3-92e6-b66bfbe9cb98"
/dev/sdb2: UUID="35327ecf-a5a7-4617-a160-1fdbfd644941" UUID_SUB="31e07d66-a656-48a8-b1fb-5b438565238e" BLOCK_SIZE="4096" TYPE="btrfs" PARTLABEL="primary" PARTUUID="93a2db85-065a-4ecf-89d4-6a8dcdb8ff99"

both have 3 subvols.
# btrfs subvolume list /mnt/test
ID 256 gen 13 top level 5 path sub1
ID 257 gen 13 top level 5 path sub2
ID 258 gen 13 top level 5 path sub3
# btrfs subvolume list /mnt/scratch
ID 256 gen 13 top level 5 path sub1
ID 257 gen 13 top level 5 path sub2
ID 258 gen 13 top level 5 path sub3


statfs.f_fsid.c is the source of 'statfs' command.

# statfs /mnt/test/sub1 /mnt/test/sub2 /mnt/test/sub3 /mnt/scratch/sub1 /mnt/scratch/sub2 /mnt/scratch/sub3
/mnt/test/sub1
        f_fsid=0x9452611458c30e57
/mnt/test/sub2
        f_fsid=0x9452611458c30e56
/mnt/test/sub3
        f_fsid=0x9452611458c30e55
/mnt/scratch/sub1
        f_fsid=0x9452611458c30e56
/mnt/scratch/sub2
        f_fsid=0x9452611458c30e57
/mnt/scratch/sub3
        f_fsid=0x9452611458c30e54

statfs.f_fsid is uniq inside a btrfs and it's subvols.
but statfs.f_fsid is NOT uniq between btrfs filesystems because just 1
bit diff of UUID.

'find /mnt/test/' and 'find /mnt/scratch/' works as expected.


Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2021/06/23


[-- Attachment #2: statfs.f_fsid.c --]
[-- Type: application/octet-stream, Size: 698 bytes --]

#define __USE_LARGEFILE64

#include <stdio.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/statfs.h>

int main(int argc, char **argv)
{
		struct statfs st;
		char *path;
		int ret=0;

		if (argc < 2) {
				fprintf(stderr, "Usage:%s path [path].. \n", argv[0]);
				return 1;
		}
		for (int i = 1; i < argc; ++i) {
				path = argv[i];
				if (statfs(path, &st) == 0 &&
					(st.f_fsid.__val[0] || st.f_fsid.__val[1])) {
						fprintf(stdout, "%s\n\tf_fsid=0x%08x%08x\n", path,
								st.f_fsid.__val[0], st.f_fsid.__val[1]);
				} else {
						fprintf(stdout, "%s\n\tstatfs error or null f_fsid.\n",path);
						ret=1;
				}
		}
		return ret;
}