Re: [PATCH 2/2 v2] xfs_repair: new secondary superblock search method] xfs_repair: new secondary superblock search method

Eric Sandeen
2016-02-10T04:49:27Z

On 2/9/16 7:34 PM, Bill O'Donnell wrote:
> Optimize secondary sb search, using similar method to find
> fs geometry as that of xfs_mkfs. If this faster method fails
> in finding a secondary sb, fall back to original brute force
> slower search.
>
> Signed-off-by: Bill O'Donnell <billodo@redhat.com>
> ---
> Makefile | 2 +-
> include/libxcmd.h | 4 +++-
> libxcmd/topology.c | 35 ++++++++++++++++++++++++++++----
> repair/Makefile | 4 ++--
> repair/sb.c | 58 ++++++++++++++++++++++++++++++++++++++++++++----------
> 5 files changed, 85 insertions(+), 18 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index fca0a42..1d60d9c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -80,7 +80,7 @@ fsr: libhandle
> growfs: libxcmd
> io: libxcmd libhandle
> quota: libxcmd
> -repair: libxlog
> +repair: libxlog libxcmd
> copy: libxlog
>
> ifeq ($(HAVE_BUILDDEFS), yes)
> diff --git a/include/libxcmd.h b/include/libxcmd.h
> index df7046e..b140adb 100644
> --- a/include/libxcmd.h
> +++ b/include/libxcmd.h
> @@ -50,6 +50,8 @@ extern int
> check_overwrite(
> char *device);
>
> -
> +extern int guess_default_geometry(__uint64_t *agsize,
> + __uint64_t *agcount,
> + libxfs_init_t x);
>
> #endif /* __LIBXCMD_H__ */
> diff --git a/libxcmd/topology.c b/libxcmd/topology.c
> index 0eeea28..b98d9b9 100644
> --- a/libxcmd/topology.c
> +++ b/libxcmd/topology.c
> @@ -192,7 +192,8 @@ out:
> return ret;
> }
>
> -static void blkid_get_topology(
> +static void
> +blkid_get_topology(
> const char *device,
> int *sunit,
> int *swidth,
> @@ -284,7 +285,8 @@ check_overwrite(
> return 1;
> }
>
> -static void blkid_get_topology(
> +static void
> +blkid_get_topology(
> const char *device,
> int *sunit,
> int *swidth,
> @@ -302,8 +304,8 @@ static void blkid_get_topology(
>
> #endif /* ENABLE_BLKID */
>
> -
> -void get_topology(
> +void
> +get_topology(
> libxfs_init_t *xi,
> struct fs_topology *ft,
> int force_overwrite)
> @@ -346,3 +348,28 @@ void get_topology(
> &lsectorsize, &psectorsize, force_overwrite);
> }
> }
> +
> +int
> +guess_default_geometry(__uint64_t *agsize, __uint64_t *agcount,
> + libxfs_init_t x)
> +{
> + struct fs_topology ft;
> + int blocklog;

When in Rome: :)

int
guess_default_geometry(
__uint64_t *agsize,
__uint64_t *agcount,
libxfs_init_t x)
{
...

(sorry, forgot to mention that last time, but other functions in the
file are laid out this way; it is easier to read)

> + __uint64_t dblocks;
> + int multidisk;
> +
> + memset(&ft, 0, sizeof(ft));
> + get_topology(&x, &ft, 1);
> +
> + /*
> + * get geometry from get_topology result.
> + * Use default block size (2^12)
> + */
> + blocklog = 12;
> + multidisk = ft.dswidth | ft.dsunit;
> + dblocks = x.dsize >> (blocklog - BBSHIFT);
> + calc_default_ag_geometry(blocklog, dblocks, multidisk,
> + agsize, agcount);
> +
> + return blocklog;
> +}
> diff --git a/repair/Makefile b/repair/Makefile
> index 251722b..d24ab1f 100644
> --- a/repair/Makefile
> +++ b/repair/Makefile
> @@ -20,8 +20,8 @@ CFILES = agheader.c attr_repair.c avl.c avl64.c bmap.c btree.c \
> progress.c prefetch.c rt.c sb.c scan.c threads.c \
> versions.c xfs_repair.c
>
> -LLDLIBS = $(LIBXFS) $(LIBXLOG) $(LIBUUID) $(LIBRT) $(LIBPTHREAD)
> -LTDEPENDENCIES = $(LIBXFS) $(LIBXLOG)
> +LLDLIBS = $(LIBBLKID) $(LIBXFS) $(LIBXLOG) $(LIBUUID) $(LIBRT) $(LIBPTHREAD) $(LIBXCMD)
> +LTDEPENDENCIES = $(LIBXFS) $(LIBXLOG) $(LIBXCMD)
> LLDFLAGS = -static-libtool-libs
>
> default: depend $(LTCOMMAND)
> diff --git a/repair/sb.c b/repair/sb.c
> index 4eef14a..8bc246e 100644
> --- a/repair/sb.c
> +++ b/repair/sb.c
> @@ -22,6 +22,7 @@
> #include "globals.h"
> #include "protos.h"
> #include "err_protos.h"
> +#include "libxcmd.h"

Nitpick, we usually include that right after libxfs.h.

> #define BSIZE (1024 * 1024)
>
> @@ -85,10 +86,11 @@ copy_sb(xfs_sb_t *source, xfs_sb_t *dest)
> }
>
> /*
> - * find a secondary superblock, copy it into the sb buffer
> + * find a secondary superblock, copy it into the sb buffer.
> + * skipsize units is bytes, it contains either the agsize in bytes
> + * (if known), or the minimum agsize in bytes if agsize unknown.
> */

Ok, so this is a little confused. There are actually 3 relevant
parameters here:

1) The place to start reading
2) The number of bytes to read at that point
3) How far to seek forward for the next read

In the old world, it's:

1) XFS_MIN_AG_BYTES
2) BSIZE (1MB)
3) BSIZE (i.e. it does sequential reads)

In the new world, it's:

1) the guessed AG size (skipsize?)
2) BSIZE (more than we need, but not expensive, so leave it)
3) the guessed AG size (skipsize)

You really have 2 different pieces of info: Where to do the first read,
and how far forward to skip for the next read. I don't think you
can convey that with a single argument, "skipsize."

> -int
> -find_secondary_sb(xfs_sb_t *rsb)
> +static int __find_secondary_sb(xfs_sb_t *rsb, __uint64_t skipsize)

better to format as:

static int
__find_secondary_sb(
xfs_sb_t *rsb,
__uint64_t skipsize)
{

but ...

> {
> xfs_off_t off;
> xfs_sb_t *sb;
> @@ -99,9 +101,9 @@ find_secondary_sb(xfs_sb_t *rsb)
> int dirty;
> int retval;
> int bsize;
> + int readsize;
>
> do_warn(_("\nattempting to find secondary superblock...\n"));
> -
> sb = (xfs_sb_t *)memalign(libxfs_device_alignment(), BSIZE);
> if (!sb) {
> do_error(
> @@ -113,13 +115,16 @@ find_secondary_sb(xfs_sb_t *rsb)
> retval = 0;
> dirty = 0;
> bsize = 0;
> + readsize = 0;
>
> /*
> * skip first sector since we know that's bad
> */
> - for (done = 0, off = XFS_AG_MIN_BYTES; !done ; off += bsize) {
> + for (done = 0, off = skipsize; !done ; off += readsize) {
> /*
> - * read disk 1 MByte at a time.

we are actually still doing that, read(x.dfd, sb, BSIZE), so I'd
leave the comment...

> + * read disk using readsize interval
> + * (either the bytecount of actual agsize or bsize if
> + * agsize undetermined.)

> */
> if (lseek64(x.dfd, off, SEEK_SET) != off) {
> done = 1;
> @@ -128,9 +133,13 @@ find_secondary_sb(xfs_sb_t *rsb)
> if (!done && (bsize = read(x.dfd, sb, BSIZE)) <= 0) {
> done = 1;
> }
> -
> + if (skipsize == XFS_AG_MIN_BYTES) {
> + readsize = bsize;
> + }
> + else {

better as:
} else {

> + readsize = skipsize;
> + }

I wouldn't call this "readsize" - we are only ever reading BSIZE at a time.
This is how much we advance each loop, so skipsize is a bit better.

But the _functional_ problem here is that it is possible and permissible
to have a filesystem with an AG size of XFS_AG_MIN_BYTES (16MB).

So if you guessed a default geometry with an agsize of XFS_AG_MIN_BYTES,
you've gone back to the fine-grained scanning. The fs probably isn't
very big as a result, so it's not the end of the world, but ...
see below.

> do_warn(".");
> -
> /*
> * check the buffer 512 bytes at a time since
> * we don't know how big the sectors really are.
> @@ -164,9 +173,38 @@ find_secondary_sb(xfs_sb_t *rsb)
> }
> }
> }
> -
> free(sb);
> - return(retval);
> + return retval;
> +}
> +
> +int
> +find_secondary_sb(xfs_sb_t *rsb)
> +{
> + int retval;
> + __uint64_t skipsize;
> + __uint64_t agcount;
> + __uint64_t agsize;
> + int blocklog;
> +
> + /*
> + * Attempt to find secondary sb with a coarse approach,
> + * using a large skipsize (agsize in bytes). Failing that,
> + * fallback to the fine-grained approach using min agsize.
> + */
> + blocklog = guess_default_geometry(&agsize, &agcount, x);
> +
> + /*
> + * use found ag geometry to quickly find secondary sb
> + */
> + skipsize = agsize << blocklog;
> + retval = __find_secondary_sb(rsb, skipsize);
> + if (!retval) {
> + /*
> + * fallback: use minimum agsize for skipsize
> + */
> + retval = __find_secondary_sb(rsb, XFS_AG_MIN_BYTES);
> + }
> + return retval;
> }

I'd make __find_secondary_sb() take (sb, start, skip) i.e. send in
this:

> + retval = __find_secondary_sb(rsb, agsize, agsize);
> + if (!retval) {
> + /*
> + * fallback: use minimum agsize for skipsize
> + */
> + retval = __find_secondary_sb(rsb, XFS_AG_MIN_BYTES, BSIZE);
> + }

and the function is something like:

static int
__find_secondary_sb(
xfs_sb_t *rsb,
xfs_off_t start,
xfs_off_t skip)

{

...

for (done = 0, off = start; !done ; off += skip) {
...
if (lseek64(x.dfd, off, SEEK_SET) != off)
done = 1;

if (!done && (read(x.dfd, sb, BSIZE)) <= 0)
done = 1;

because you really can't deduce both the starting point and the skip-ahead
size from just one parameter.

-Eric