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Varad Gautam authored
d_delete only unhashes an entry if it is reached with dentry->d_lockref.count != 1. Prior to commit 8ead9dd5 ("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 8ead9dd5 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: 8ead9dd5 ("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>
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