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authorVarad Gautam <vrd@amazon.de>2019-01-24 14:03:06 +0100
committerAl Viro <viro@zeniv.linux.org.uk>2019-01-24 13:38:30 -0500
commit73052b0daee0b750b39af18460dfec683e4f5887 (patch)
tree8f9768f12fd6768fbe89ab5cf7033f7bbada19c8
parentbfeffd155283772bbe78c6a05dec7c0128ee500c (diff)
fs/devpts: always delete dcache dentry-s in dput()
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 <vrd@amazon.de> Reported-by: Zheng Wang <wanz@amazon.de> Reported-by: Brandon Schwartz <bsschwar@amazon.de> Root-caused-by: Maximilian Heyne <mheyne@amazon.de> Root-caused-by: Nicolas Pernas Maradei <npernas@amazon.de> CC: David Woodhouse <dwmw@amazon.co.uk> CC: Maximilian Heyne <mheyne@amazon.de> CC: Stefan Nuernberger <snu@amazon.de> CC: Amit Shah <aams@amazon.de> CC: Linus Torvalds <torvalds@linux-foundation.org> CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org> CC: Al Viro <viro@ZenIV.linux.org.uk> CC: Christian Brauner <christian.brauner@ubuntu.com> CC: Eric W. Biederman <ebiederm@xmission.com> CC: Matthew Wilcox <willy@infradead.org> CC: Eric Biggers <ebiggers@google.com> CC: <stable@vger.kernel.org> # 4.9+ Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
-rw-r--r--fs/devpts/inode.c1
1 files changed, 1 insertions, 0 deletions
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index c53814539070..553a3f3300ae 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -455,6 +455,7 @@ 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;