xfs: use generic percpu counters for inode counter

XFS has hand-rolled per-cpu counters for the superblock since before
there was any generic implementation. There are some warts around
the  use of them for the inode counter as the hand rolled counter is
designed to be accurate at zero, but has no specific accurracy at
any other value. This design causes problems for the maximum inode
count threshold enforcement, as there is no trigger that balances
the counters as they get close tothe maximum threshold.

Instead of designing new triggers for balancing, just replace the
handrolled per-cpu counter with a generic counter.  This enables us
to update the counter through the normal superblock modification
funtions, but rather than do that we add a xfs_mod_icount() helper
function (from Christoph Hellwig) and keep the percpu counter
outside the superblock in the struct xfs_mount.

This means we still need to initialise the per-cpu counter
specifically when we read the superblock, and vice versa when we
log/write it, but it does mean that we don't need to change any
other code.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 8fcc4cc..0aa4428 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1085,6 +1085,7 @@ xfs_fs_statfs(
 	xfs_sb_t		*sbp = &mp->m_sb;
 	struct xfs_inode	*ip = XFS_I(dentry->d_inode);
 	__uint64_t		fakeinos, id;
+	__uint64_t		icount;
 	xfs_extlen_t		lsize;
 	__int64_t		ffree;
 
@@ -1096,6 +1097,7 @@ xfs_fs_statfs(
 	statp->f_fsid.val[1] = (u32)(id >> 32);
 
 	xfs_icsb_sync_counters(mp, XFS_ICSB_LAZY_COUNT);
+	icount = percpu_counter_sum(&mp->m_icount);
 
 	spin_lock(&mp->m_sb_lock);
 	statp->f_bsize = sbp->sb_blocksize;
@@ -1104,8 +1106,7 @@ xfs_fs_statfs(
 	statp->f_bfree = statp->f_bavail =
 				sbp->sb_fdblocks - XFS_ALLOC_SET_ASIDE(mp);
 	fakeinos = statp->f_bfree << sbp->sb_inopblog;
-	statp->f_files =
-	    MIN(sbp->sb_icount + fakeinos, (__uint64_t)XFS_MAXINUMBER);
+	statp->f_files = MIN(icount + fakeinos, (__uint64_t)XFS_MAXINUMBER);
 	if (mp->m_maxicount)
 		statp->f_files = min_t(typeof(statp->f_files),
 					statp->f_files,
@@ -1117,7 +1118,7 @@ xfs_fs_statfs(
 					sbp->sb_icount);
 
 	/* make sure statp->f_ffree does not underflow */
-	ffree = statp->f_files - (sbp->sb_icount - sbp->sb_ifree);
+	ffree = statp->f_files - (icount - sbp->sb_ifree);
 	statp->f_ffree = max_t(__int64_t, ffree, 0);
 
 	spin_unlock(&mp->m_sb_lock);