From 8c93709f25e738b6cfea72eda0a0593365f049ea Mon Sep 17 00:00:00 2001 From: Varad Gautam Date: Thu, 24 Jan 2019 14:03:06 +0100 Subject: fs/devpts: always delete dcache dentry-s in dput() commit 73052b0daee0b750b39af18460dfec683e4f5887 upstream. d_delete only unhashes an entry if it is reached with dentry->d_lockref.count != 1. Prior to commit 8ead9dd54716 ("devpts: more pty driver interface cleanups"), d_delete was called on a dentry from devpts_pty_kill with two references held, which would trigger the unhashing, and the subsequent dputs would release it. Commit 8ead9dd54716 reworked devpts_pty_kill to stop acquiring the second reference from d_find_alias, and the d_delete call left the dentries still on the hashed list without actually ever being dropped from dcache before explicit cleanup. This causes the number of negative dentries for devpts to pile up, and an `ls /dev/pts` invocation can take seconds to return. Provide always_delete_dentry() from simple_dentry_operations as .d_delete for devpts, to make the dentry be dropped from dcache. Without this cleanup, the number of dentries in /dev/pts/ can be grown arbitrarily as: `python -c 'import pty; pty.spawn(["ls", "/dev/pts"])'` A systemtap probe on dcache_readdir to count d_subdirs shows this count to increase with each pty spawn invocation above: probe kernel.function("dcache_readdir") { subdirs = &@cast($file->f_path->dentry, "dentry")->d_subdirs; p = subdirs; p = @cast(p, "list_head")->next; i = 0 while (p != subdirs) { p = @cast(p, "list_head")->next; i = i+1; } printf("number of dentries: %d\n", i); } Fixes: 8ead9dd54716 ("devpts: more pty driver interface cleanups") Signed-off-by: Varad Gautam Reported-by: Zheng Wang Reported-by: Brandon Schwartz Root-caused-by: Maximilian Heyne Root-caused-by: Nicolas Pernas Maradei CC: David Woodhouse CC: Maximilian Heyne CC: Stefan Nuernberger CC: Amit Shah CC: Linus Torvalds CC: Greg Kroah-Hartman CC: Al Viro CC: Christian Brauner CC: Eric W. Biederman CC: Matthew Wilcox CC: Eric Biggers CC: # 4.9+ Signed-off-by: Al Viro Signed-off-by: Greg Kroah-Hartman --- fs/devpts/inode.c | 1 + 1 file changed, 1 insertion(+) (limited to 'fs/devpts') diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c index 108df2e3602c..81be3ba17cbf 100644 --- a/fs/devpts/inode.c +++ b/fs/devpts/inode.c @@ -396,6 +396,7 @@ devpts_fill_super(struct super_block *s, void *data, int silent) s->s_blocksize_bits = 10; s->s_magic = DEVPTS_SUPER_MAGIC; s->s_op = &devpts_sops; + s->s_d_op = &simple_dentry_operations; s->s_time_gran = 1; error = -ENOMEM; -- cgit v1.2.3