On Thu, Jun 17, 2010 at 02:54:01PM +1000, Neil Brown wrote:
>
> If you export two subvolumes of a btrfs filesystem, they will both be
> given the same uuid so lookups will be confused.
> blkid cannot differentiate the two, so we must use the fsid from
> statfs64 to identify the filesystem.
>
> We cannot tell if blkid or statfs is best without knowing internal
> details of the filesystem in question, so we need to encode specific
> knowledge of btrfs in mountd. This is unfortunate.
>
> To ensure smooth handling of this and possible future changes in uuid
> generation, we add infrastructure for multiple different uuids to be
> recognised on old filehandles, but only the preferred on is used on
> new filehandles.
Could you just contatenate the two (or hash them somehow)? Or does that
just use up too much space in the filehandle?
--b.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
>
> --
> This is a substantially revised version of a patch I posted a while
> ago.
> I tried to find a way to do it would hard coding knowledge of btrfs in
> nfs-utils, but it isn't possible. For some filesystems, f_fsid is
> best, for some it is worst. No way to tell the difference.
>
> This patch add infrastructure so that if we find a better way to get a
> good uuid (e.g. a new syscall), we can slot it in for new filehandles,
> but old filehandles using the old uuid will still work.
>
> I believe this is ready for inclusion upstream.
> Thanks,
> NeilBrown
>
>
> diff --git a/utils/mountd/cache.c b/utils/mountd/cache.c
> index caef5b2..85cd829 100644
> --- a/utils/mountd/cache.c
> +++ b/utils/mountd/cache.c
> @@ -170,13 +170,16 @@ void auth_unix_gid(FILE *f)
> #if USE_BLKID
> static const char *get_uuid_blkdev(char *path)
> {
> + /* We set *safe if we know that we need the
> + * fsid from statfs too.
> + */
> static blkid_cache cache = NULL;
> struct stat stb;
> char *devname;
> blkid_tag_iterate iter;
> blkid_dev dev;
> const char *type;
> - const char *val = NULL;
> + const char *val, *uuid = NULL;
>
> if (cache == NULL)
> blkid_get_cache(&cache, NULL);
> @@ -193,42 +196,29 @@ static const char *get_uuid_blkdev(char *path)
> iter = blkid_tag_iterate_begin(dev);
> if (!iter)
> return NULL;
> - while (blkid_tag_next(iter, &type, &val) == 0)
> + while (blkid_tag_next(iter, &type, &val) == 0) {
> if (strcmp(type, "UUID") == 0)
> + uuid = val;
> + if (strcmp(type, "TYPE") == 0 &&
> + strcmp(val, "btrfs") == 0) {
> + uuid = NULL;
> break;
> + }
> + }
> blkid_tag_iterate_end(iter);
> - return val;
> + return uuid;
> }
> #else
> #define get_uuid_blkdev(path) (NULL)
> #endif
>
> -int get_uuid(char *path, char *uuid, int uuidlen, char *u)
> +int get_uuid(const char *val, int uuidlen, char *u)
> {
> /* extract hex digits from uuidstr and compose a uuid
> * of the given length (max 16), xoring bytes to make
> - * a smaller uuid. Then compare with uuid
> + * a smaller uuid.
> */
> int i = 0;
> - const char *val = NULL;
> - char fsid_val[17];
> -
> - if (path) {
> - val = get_uuid_blkdev(path);
> - if (!val) {
> - struct statfs64 st;
> -
> - if (statfs64(path, &st))
> - return 0;
> - if (!st.f_fsid.__val[0] && !st.f_fsid.__val[1])
> - return 0;
> - snprintf(fsid_val, 17, "%08x%08x",
> - st.f_fsid.__val[0], st.f_fsid.__val[1]);
> - val = fsid_val;
> - }
> - } else {
> - val = uuid;
> - }
>
> memset(u, 0, uuidlen);
> for ( ; *val ; val++) {
> @@ -252,6 +242,60 @@ int get_uuid(char *path, char *uuid, int uuidlen, char *u)
> return 1;
> }
>
> +int uuid_by_path(char *path, int type, int uuidlen, char *uuid)
> +{
> + /* get a uuid for the filesystem found at 'path'.
> + * There are several possible ways of generating the
> + * uuids (types).
> + * Type 0 is used for new filehandles, while other types
> + * may be used to interpret old filehandle - to ensure smooth
> + * forward migration.
> + * We return 1 if a uuid was found (and it might be worth
> + * trying the next type) or 0 if no more uuid types can be
> + * extracted.
> + */
> +
> + /* Possible sources of uuid are
> + * - blkid uuid
> + * - statfs64 uuid
> + *
> + * On some filesystems (e.g. vfat) the statfs64 uuid is simply an
> + * encoding of the device that the filesystem is mounted from, so
> + * it we be very bad to use that (as device numbers change). blkid
> + * must be preferred.
> + * On other filesystems (e.g. btrfs) the statfs64 uuid contains
> + * important info that the blkid uuid cannot contain: This happens
> + * when multiple subvolumes are exported (they have the same
> + * blkid uuid but different statfs64 uuids).
> + * We rely on get_uuid_blkdev *knowing* which is which and not returning
> + * a uuid for filesystems where the statfs64 uuid is better.
> + *
> + */
> + struct statfs64 st;
> + char fsid_val[17];
> + const char *blkid_val;
> + const char *val;
> +
> + blkid_val = get_uuid_blkdev(path);
> +
> + if (statfs64(path, &st) == 0 &&
> + (st.f_fsid.__val[0] || st.f_fsid.__val[1]))
> + snprintf(fsid_val, 17, "%08x%08x",
> + st.f_fsid.__val[0], st.f_fsid.__val[1]);
> + else
> + fsid_val[0] = 0;
> +
> + if (blkid_val && (type--) == 0)
> + val = blkid_val;
> + else if (fsid_val[0] && (type--) == 0)
> + val = fsid_val;
> + else
> + return 0;
> +
> + get_uuid(val, uuidlen, uuid);
> + return 1;
> +}
> +
> /* Iterate through /etc/mtab, finding mountpoints
> * at or below a given path
> */
> @@ -398,6 +442,7 @@ void nfsd_fh(FILE *f)
> struct stat stb;
> char u[16];
> char *path;
> + int type;
>
> if (exp->m_export.e_flags & NFSEXP_CROSSMOUNT) {
> static nfs_export *prev = NULL;
> @@ -461,10 +506,14 @@ void nfsd_fh(FILE *f)
> continue;
> check_uuid:
> if (exp->m_export.e_uuid)
> - get_uuid(NULL, exp->m_export.e_uuid,
> + get_uuid(exp->m_export.e_uuid,
> uuidlen, u);
> - else if (get_uuid(path, NULL, uuidlen, u) == 0)
> - continue;
> + else
> + for (type = 0;
> + uuid_by_path(path, type, uuidlen, u);
> + type++)
> + if (memcmp(u, fhuuid, uuidlen) != 0)
> + break;
>
> if (memcmp(u, fhuuid, uuidlen) != 0)
> continue;
> @@ -600,13 +649,13 @@ static int dump_to_cache(FILE *f, char *domain, char *path, struct exportent *ex
> write_secinfo(f, exp, flag_mask);
> if (exp->e_uuid == NULL || different_fs) {
> char u[16];
> - if (get_uuid(path, NULL, 16, u)) {
> + if (uuid_by_path(path, 0, 16, u)) {
> qword_print(f, "uuid");
> qword_printhex(f, u, 16);
> }
> } else {
> char u[16];
> - get_uuid(NULL, exp->e_uuid, 16, u);
> + get_uuid(exp->e_uuid, 16, u);
> qword_print(f, "uuid");
> qword_printhex(f, u, 16);
> }
> --
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