[PATCH 3.14 02/29] freeing unlinked file indefinitely delayed

From: Greg Kroah-Hartman
Date: Sat Aug 08 2015 - 18:46:38 EST

3.14-stable review patch. If anyone has any objections, please let me know.


From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 75a6f82a0d10ef8f13cd8fe7212911a0252ab99e upstream.

Normally opening a file, unlinking it and then closing will have
the inode freed upon close() (provided that it's not otherwise busy and
has no remaining links, of course). However, there's one case where that
does *not* happen. Namely, if you open it by fhandle with cold dcache,
then unlink() and close().

In normal case you get d_delete() in unlink(2) notice that dentry
is busy and unhash it; on the final dput() it will be forcibly evicted from
dcache, triggering iput() and inode removal. In this case, though, we end
up with *two* dentries - disconnected (created by open-by-fhandle) and
regular one (used by unlink()). The latter will have its reference to inode
dropped just fine, but the former will not - it's considered hashed (it
is on the ->s_anon list), so it will stay around until the memory pressure
will finally do it in. As the result, we have the final iput() delayed
indefinitely. It's trivial to reproduce -

void flush_dcache(void)
system("mount -o remount,rw /");

static char buf[20 * 1024 * 1024];

int fd;
union {
struct file_handle f;
char buf[MAX_HANDLE_SZ];
} x;
int m;

x.f.handle_bytes = sizeof(x);
mkdir("foo", 0700);
fd = open("foo/bar", O_CREAT | O_RDWR, 0600);
name_to_handle_at(AT_FDCWD, "foo/bar", &x.f, &m, 0);
fd = open_by_handle_at(AT_FDCWD, &x.f, O_RDWR);
write(fd, buf, sizeof(buf));
system("df ."); /* 20Mb eaten */
system("df ."); /* should've freed those 20Mb */
system("df ."); /* should be the same as #2 */

will spit out something like
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/root 322023 303843 1131 100% /
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/root 322023 303843 1131 100% /
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/root 322023 283282 21692 93% /
- inode gets freed only when dentry is finally evicted (here we trigger
than by remount; normally it would've happened in response to memory
pressure hell knows when).

Acked-by: J. Bruce Fields <bfields@xxxxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

fs/dcache.c | 3 +++
1 file changed, 3 insertions(+)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -587,6 +587,9 @@ repeat:
if (unlikely(d_unhashed(dentry)))
goto kill_it;

+ if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
+ goto kill_it;
if (unlikely(dentry->d_flags & DCACHE_OP_DELETE)) {
if (dentry->d_op->d_delete(dentry))
goto kill_it;

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