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Re: File system block reservation mechanism is broken

To: Shailendra Tripathi <stripathi@xxxxxxxxx>
Subject: Re: File system block reservation mechanism is broken
From: Stephane Doyon <sdoyon@xxxxxxxxx>
Date: Mon, 18 Sep 2006 11:24:44 -0400 (EDT)
Cc: xfs@xxxxxxxxxxx
In-reply-to: <450EB500.3070000@xxxxxxxxx>
References: <Pine.LNX.4.64.0609151242110.21194@xxxxxxxxxxxxxxxxxxxxx> <450EB500.3070000@xxxxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
On Mon, 18 Sep 2006, Shailendra Tripathi wrote:

Hi Stephane,

 The code in xfs_reserve_blocks() just locks the superblock and then
 consults and modifies mp->m_sb.sb_fdblocks. However, if the per-cpu
 counter is active, the count is spread across the per-cpu counters, and
 the superblock field does not contain an accurate count, nor does
 modifying it have any effect.

This is the fast path. However, there is slow path where it actually falls back to the earlier mechanism where global lock (spin-lock) is held and then counters are guranteed to be consistent. The goal is not to take the

AFAICT, xfs_reserve_blocks() is only aware of the old / slow path way.

please clarify little more as to why it fails. I can't see the problem.

int
xfs_reserve_blocks(...)
{
...
        s = XFS_SB_LOCK(mp);
...
                lcounter = mp->m_sb.sb_fdblocks - delta;
...
                        mp->m_sb.sb_fdblocks = lcounter;
...
}

It assumes the superblock counters are current and accurate, and that they are authoritative... it hasn't been converted to use the new fast path, it always uses the slow path.

Most of the time (except when some CPU's counter has drained), the fdblocks count will be in the per-cpu mp->m_sb_cnts array of counters. m_sb.sb_fdblocks only contains the value left in it last time we totaled the counters. Modifying m_sb.sb_fdblocks does nothing because that value is known to be inaccurate and will be recalculated and overwritten next time the slow path to that counter is used.

Does that make it clearer?

Stephane Doyon wrote:
 [Resending. Seems my previous post did not make it somehow...]

 The mechanism allowing to reserve file system blocks, xfs_reserve_blocks()
 / XFS_IOC_SET_RESBLKS, appears to have been broken by the patch that
 introduced per-cpu superblock counters.


 The observed behavior is that xfs_io -xc "resblks <nnn>" <file> has no
 effect: the resblks does get set and can be retrieved, but the free blocks
 count does not decrease.

 The XFS_SET_ASIDE_BLOCKS, introduced in the recent full file system
 deadlock fix, probably also needs to be taken into account.

 I'm not particularly familiar with the code, but AFAICT something along
 the lines of the following patch should fix it.

 Signed-off-by: Stephane Doyon <sdoyon@xxxxxxxxx>

 Index: linux/fs/xfs/xfs_fsops.c
 ===================================================================
 --- linux.orig/fs/xfs/xfs_fsops.c    2006-09-13 11:31:36.000000000 -0400
 +++ linux/fs/xfs/xfs_fsops.c    2006-09-13 11:32:06.782591491 -0400
 @@ -505,6 +505,7 @@

       request = *inval;
       s = XFS_SB_LOCK(mp);
 +    xfs_icsb_disable_counter(mp, XFS_SBS_FDBLOCKS);

       /*
       * If our previous reservation was larger than the current value,
 @@ -520,14 +521,14 @@
           mp->m_resblks = request;
       } else {
          delta = request - mp->m_resblks;
 -        lcounter = mp->m_sb.sb_fdblocks - delta;
 +        lcounter = mp->m_sb.sb_fdblocks - XFS_SET_ASIDE_BLOCKS(mp) -
 delta;
           if (lcounter < 0) {
              /* We can't satisfy the request, just get what we can */
 -            mp->m_resblks += mp->m_sb.sb_fdblocks;
 -            mp->m_resblks_avail += mp->m_sb.sb_fdblocks;
 -            mp->m_sb.sb_fdblocks = 0;
 +            mp->m_resblks += mp->m_sb.sb_fdblocks -
 XFS_SET_ASIDE_BLOCKS(mp);
 +            mp->m_resblks_avail += mp->m_sb.sb_fdblocks -
 XFS_SET_ASIDE_BLOCKS(mp);
 +            mp->m_sb.sb_fdblocks = XFS_SET_ASIDE_BLOCKS(mp);
          } else {
 -            mp->m_sb.sb_fdblocks = lcounter;
 +            mp->m_sb.sb_fdblocks = lcounter + XFS_SET_ASIDE_BLOCKS(mp);
               mp->m_resblks = request;
               mp->m_resblks_avail += delta;
          }
 Index: linux/fs/xfs/xfs_mount.c
 ===================================================================
 --- linux.orig/fs/xfs/xfs_mount.c    2006-09-13 11:31:36.000000000 -0400
 +++ linux/fs/xfs/xfs_mount.c    2006-09-13 11:32:06.784591724 -0400
 @@ -58,7 +58,6 @@
                           int, int);
   STATIC int    xfs_icsb_modify_counters_locked(xfs_mount_t *,
 xfs_sb_field_t,
                          int, int);
 -STATIC int    xfs_icsb_disable_counter(xfs_mount_t *, xfs_sb_field_t);

  #else

 @@ -1254,26 +1253,6 @@
   }

 /*
 - * In order to avoid ENOSPC-related deadlock caused by
 - * out-of-order locking of AGF buffer (PV 947395), we place
 - * constraints on the relationship among actual allocations for
 - * data blocks, freelist blocks, and potential file data bmap
 - * btree blocks. However, these restrictions may result in no
 - * actual space allocated for a delayed extent, for example, a data
 - * block in a certain AG is allocated but there is no additional
 - * block for the additional bmap btree block due to a split of the
 - * bmap btree of the file. The result of this may lead to an
 - * infinite loop in xfssyncd when the file gets flushed to disk and
 - * all delayed extents need to be actually allocated. To get around
 - * this, we explicitly set aside a few blocks which will not be
 - * reserved in delayed allocation. Considering the minimum number of
 - * needed freelist blocks is 4 fsbs _per AG_, a potential split of file's
 bmap
 - * btree requires 1 fsb, so we set the number of set-aside blocks
 - * to 4 + 4*agcount.
 - */
 -#define XFS_SET_ASIDE_BLOCKS(mp)  (4 + ((mp)->m_sb.sb_agcount * 4))
 -
 -/*
    * xfs_mod_incore_sb_unlocked() is a utility routine common used to
    apply
    * a delta to a specified field in the in-core superblock.  Simply
    * switch on the field indicated and apply the delta to that field.
 @@ -1906,7 +1885,7 @@
       return test_bit(field, &mp->m_icsb_counters);
   }

 -STATIC int
 +int
   xfs_icsb_disable_counter(
       xfs_mount_t    *mp,
       xfs_sb_field_t    field)
 Index: linux/fs/xfs/xfs_mount.h
 ===================================================================
 --- linux.orig/fs/xfs/xfs_mount.h    2006-09-13 11:31:36.000000000 -0400
 +++ linux/fs/xfs/xfs_mount.h    2006-09-13 11:33:24.441557999 -0400
 @@ -307,10 +307,14 @@

   extern int    xfs_icsb_init_counters(struct xfs_mount *);
   extern void    xfs_icsb_sync_counters_lazy(struct xfs_mount *);
 +/* Can't forward declare typedefs... */
 +struct xfs_mount;
 +extern int xfs_icsb_disable_counter(struct xfs_mount *, xfs_sb_field_t);

#  else
#  define xfs_icsb_init_counters(mp)    (0)
#  define xfs_icsb_sync_counters_lazy(mp)    do { } while (0)
 +#define xfs_icsb_disable_counters(mp, field)    do { } while (0)
   #endif

  typedef struct xfs_mount {
 @@ -574,6 +578,27 @@
#  define    XFS_SB_LOCK(mp)        mutex_spinlock(&(mp)->m_sb_lock)
#  define    XFS_SB_UNLOCK(mp,s)    mutex_spinunlock(&(mp)->m_sb_lock,(s))

 +
 +/*
 + * In order to avoid ENOSPC-related deadlock caused by
 + * out-of-order locking of AGF buffer (PV 947395), we place
 + * constraints on the relationship among actual allocations for
 + * data blocks, freelist blocks, and potential file data bmap
 + * btree blocks. However, these restrictions may result in no
 + * actual space allocated for a delayed extent, for example, a data
 + * block in a certain AG is allocated but there is no additional
 + * block for the additional bmap btree block due to a split of the
 + * bmap btree of the file. The result of this may lead to an
 + * infinite loop in xfssyncd when the file gets flushed to disk and
 + * all delayed extents need to be actually allocated. To get around
 + * this, we explicitly set aside a few blocks which will not be
 + * reserved in delayed allocation. Considering the minimum number of
 + * needed freelist blocks is 4 fsbs _per AG_, a potential split of file's
 bmap
 + * btree requires 1 fsb, so we set the number of set-aside blocks
 + * to 4 + 4*agcount.
 + */
 +#define XFS_SET_ASIDE_BLOCKS(mp)  (4 + ((mp)->m_sb.sb_agcount * 4))
 +
   extern xfs_mount_t *xfs_mount_init(void);
   extern void    xfs_mod_sb(xfs_trans_t *, __int64_t);
   extern void    xfs_mount_free(xfs_mount_t *mp, int remove_bhv);







--
Stéphane Doyon
Software Developer
Maximum Throughput Inc.
http://www.max-t.com
sdoyon@xxxxxxxxx
Phone: (514) 938-7297
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