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Varad Gautam 73052b0dae 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 8ead9dd547 ("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 8ead9dd547 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: 8ead9dd547 ("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>
 		2019-01-24 13:38:30 -05:00
Documentation Kbuild late updates for v4.21 2019-01-06 16:33:10 -08:00
LICENSES This is a fairly typical cycle for documentation. There's some welcome 2018-10-24 18:01:11 +01:00
arch Kbuild late updates for v4.21 2019-01-06 16:33:10 -08:00
block for-4.21/block-20190102 2019-01-02 18:49:58 -08:00
certs kbuild: remove redundant target cleaning on failure 2019-01-06 09:46:51 +09:00
crypto Kconfig updates for v4.21 2018-12-29 13:03:29 -08:00
drivers chrome platform changes for v4.21 2019-01-06 11:40:06 -08:00
firmware kbuild: change filechk to surround the given command with { } 2019-01-06 09:46:51 +09:00
fs fs/devpts: always delete dcache dentry-s in dput() 2019-01-24 13:38:30 -05:00
include Kbuild late updates for v4.21 2019-01-06 16:33:10 -08:00
init jump_label: move 'asm goto' support test to Kconfig 2019-01-06 09:46:51 +09:00
ipc ipc: IPCMNI limit check for semmni 2018-10-31 08:54:14 -07:00
kernel Kbuild late updates for v4.21 2019-01-06 16:33:10 -08:00
lib Kbuild late updates for v4.21 2019-01-06 16:33:10 -08:00
mm Change mincore() to count "mapped" pages rather than "cached" pages 2019-01-06 13:43:02 -08:00
net jump_label: move 'asm goto' support test to Kconfig 2019-01-06 09:46:51 +09:00
samples Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/hid/hid 2019-01-05 17:53:40 -08:00
scripts kconfig: rename generated .*conf-cfg to *conf-cfg 2019-01-06 10:47:09 +09:00
security Merge branch 'mount.part1' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-01-05 13:25:58 -08:00
sound Remove 'type' argument from access_ok() function 2019-01-03 18:57:57 -08:00
tools Kbuild late updates for v4.21 2019-01-06 16:33:10 -08:00
usr user/Makefile: Fix typo and capitalization in comment section 2018-12-11 00:18:03 +09:00
virt Merge branch 'akpm' (patches from Andrew) 2019-01-05 09:16:18 -08:00
.clang-format page cache: Convert find_get_pages_contig to XArray 2018-10-21 10:46:34 -04:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Add hch to .get_maintainer.ignore 2015-08-21 14:30:10 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore kbuild: Add support for DT binding schema checks 2018-12-13 09:41:32 -06:00
.mailmap A few early MIPS fixes for 4.21: 2019-01-05 12:48:25 -08:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS Add CREDITS entry for Shaohua Li 2019-01-04 14:27:09 -07:00
Kbuild kbuild: use assignment instead of define ... endef for filechk_* rules 2019-01-06 10:22:35 +09:00
Kconfig kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt 2018-08-02 08:06:55 +09:00
MAINTAINERS chrome platform changes for v4.21 2019-01-06 11:40:06 -08:00
Makefile Linux 5.0-rc1 2019-01-06 17:08:20 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.