fs/devpts: always delete dcache dentry-s in dput()
diff mbox series

Message ID 1548334986-28667-1-git-send-email-vrd@amazon.com
State Accepted
Commit 73052b0daee0b750b39af18460dfec683e4f5887
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Series
  • fs/devpts: always delete dcache dentry-s in dput()
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Commit Message

vrd@amazon.com Jan. 24, 2019, 1:03 p.m. UTC
From: Varad Gautam <vrd@amazon.de>

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+
---
 fs/devpts/inode.c | 1 +
 1 file changed, 1 insertion(+)

Comments

Al Viro Jan. 24, 2019, 6:20 p.m. UTC | #1
On Thu, Jan 24, 2019 at 02:03:06PM +0100, vrd@amazon.com wrote:

> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index c538145..553a3f3 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;

Applied.

Patch
diff mbox series

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index c538145..553a3f3 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;