[PATCH V2] xfs: be honest about used inodes in statfs

[Eric Sandeen]

Because we have lazy counters, it's possible that we over-allocate
inodes past the maxicount (imaxpct) limit.  

A previous commit,

 2fe3366 xfs: ensure f_ffree returned by statfs() is non-negative

stopped statfs from underflowing f_ffree in this case, but that
only happened when we mis-reported f_files, capped at maxicount.

Change statfs to report the actual number of inodes allocated,
even if it is greater than maxicount.  It's reality.
Deal with it.  

(New clearer code flow thanks to Brian!)

Logic-made-readable-by: Brian Foster <bfoster at redhat.com>
Signed-off-by: Eric Sandeen <sandeen at redhat.com>
---

V2: Use Brian's suggested logic for working out the numbers

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index f317488..0dbcc17 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1083,7 +1083,6 @@ xfs_fs_statfs(
 	struct xfs_inode	*ip = XFS_I(dentry->d_inode);
 	__uint64_t		fakeinos, id;
 	xfs_extlen_t		lsize;
-	__int64_t		ffree;
 
 	statp->f_type = XFS_SB_MAGIC;
 	statp->f_namelen = MAXNAMELEN - 1;
@@ -1100,17 +1099,19 @@ xfs_fs_statfs(
 	statp->f_blocks = sbp->sb_dblocks - lsize;
 	statp->f_bfree = statp->f_bavail =
 				sbp->sb_fdblocks - XFS_ALLOC_SET_ASIDE(mp);
+	/*
+	 * Potential number of new inodes in free blocks, limited by maxicount.
+	 */
 	fakeinos = statp->f_bfree << sbp->sb_inopblog;
-	statp->f_files =
-	    MIN(sbp->sb_icount + fakeinos, (__uint64_t)XFS_MAXINUMBER);
 	if (mp->m_maxicount)
-		statp->f_files = min_t(typeof(statp->f_files),
-					statp->f_files,
-					mp->m_maxicount);
+		fakeinos = mp->m_maxicount > sbp->sb_icount ?
+			   MIN(mp->m_maxicount - sbp->sb_icount, fakeinos) : 0;
+
+	/* Total possible files is current inodes + potential new inodes */
+	statp->f_files = MIN(sbp->sb_icount + fakeinos,
+			     (__uint64_t) XFS_MAXINUMBER);
 
-	/* make sure statp->f_ffree does not underflow */
-	ffree = statp->f_files - (sbp->sb_icount - sbp->sb_ifree);
-	statp->f_ffree = max_t(__int64_t, ffree, 0);
+	statp->f_ffree = statp->f_files - (sbp->sb_icount - sbp->sb_ifree);
 
 	spin_unlock(&mp->m_sb_lock);