[PATCH] tmpfs: truncate prealloc blocks past i_size

[Hugh Dickins]

From: Josef Bacik <jbacik@xxxxxx>

One of the rocksdb people noticed that when you do something like this

fallocate(fd, FALLOC_FL_KEEP_SIZE, 0, 10M)
pwrite(fd, buf, 5M, 0)
ftruncate(5M)

on tmpfs, the file would still take up 10M: which led to super fun issues
because we were getting ENOSPC before we thought we should be getting
ENOSPC.  This patch fixes the problem, and mirrors what all the other
fs'es do (and was agreed to be the correct behaviour at LSF).

I tested it locally to make sure it worked properly with the following

xfs_io -f -c "falloc -k 0 10M" -c "pwrite 0 5M" -c "truncate 5M" file

Without the patch we have "Blocks: 20480", with the patch we have the
correct value of "Blocks: 10240".

Signed-off-by: Josef Bacik <jbacik@xxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -569,7 +569,7 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 			i_size_write(inode, newsize);
 			inode->i_ctime = inode->i_mtime = CURRENT_TIME;
 		}
-		if (newsize < oldsize) {
+		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);
-- 
1.8.3.1
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