tmpfs: truncate prealloc blocks past i_size

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@fb.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 3759099d8ce438f57398d84f50049b9e8821bd3a..4caf8ed24d6586e32ab910f28f945c01cef6373b 100644 (file)
--- 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);