tmpfs: avoid a little creat and stat slowdown

LKP reports that v4.2 commit afa2db2f ("tmpfs: truncate prealloc
blocks past i_size") causes a 14.5% slowdown in the AIM9 creat-clo
benchmark.

creat-clo does just what you'd expect from the name, and creat's O_TRUNC
on 0-length file does indeed get into more overhead now shmem_setattr()
tests "0 <= 0" instead of "0 < 0".

I'm not sure how much we care, but I think it would not be too VW-like to
add in a check for whether any pages (or swap) are allocated: if none are
allocated, there's none to remove from the radix_tree.  At first I thought
that check would be good enough for the unmaps too, but no: we should not
skip the unlikely case of unmapping pages beyond the new EOF, which were
COWed from holes which have now been reclaimed, leaving none.

This gives me an 8.5% speedup: on Haswell instead of LKP's Westmere, and
running a debug config before and after: I hope those account for the
lesser speedup.

And probably someone has a benchmark where a thousand threads keep on
stat'ing the same file repeatedly: forestall that report by adjusting v4.3
commit 44a30220 ("shmem: recalculate file inode when fstat") not to
take the spinlock in shmem_getattr() when there's no work to do.
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Ying Huang <ying.huang@linux.intel.com>
Tested-by: Ying Huang <ying.huang@linux.intel.com>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -548,12 +548,12 @@ static int shmem_getattr(struct vfsmount *mnt, struct dentry *dentry,
 	struct inode *inode = dentry->d_inode;
 	struct shmem_inode_info *info = SHMEM_I(inode);
 
+	if (info->alloced - info->swapped != inode->i_mapping->nrpages) {
 		spin_lock(&info->lock);
 		shmem_recalc_inode(inode);
 		spin_unlock(&info->lock);
-
+	}
 	generic_fillattr(inode, stat);
-
 	return 0;
 }
 
@@ -586,10 +586,16 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 		}
 		if (newsize <= oldsize) {
 			loff_t holebegin = round_up(newsize, PAGE_SIZE);
-			unmap_mapping_range(inode->i_mapping, holebegin, 0, 1);
-			shmem_truncate_range(inode, newsize, (loff_t)-1);
+			if (oldsize > holebegin)
+				unmap_mapping_range(inode->i_mapping,
+							holebegin, 0, 1);
+			if (info->alloced)
+				shmem_truncate_range(inode,
+							newsize, (loff_t)-1);
 			/* unmap again to remove racily COWed private pages */
-			unmap_mapping_range(inode->i_mapping, holebegin, 0, 1);
+			if (oldsize > holebegin)
+				unmap_mapping_range(inode->i_mapping,
+							holebegin, 0, 1);
 		}
 	}