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Josef Bacik 3fe2895cfe mm: fix page leak with multiple threads mapping the same page
We have an application with a lot of threads that use a shared mmap backed
by tmpfs mounted with -o huge=within_size.  This application started
leaking loads of huge pages when we upgraded to a recent kernel.

Using the page ref tracepoints and a BPF program written by Tejun Heo we
were able to determine that these pages would have multiple refcounts from
the page fault path, but when it came to unmap time we wouldn't drop the
number of refs we had added from the faults.

I wrote a reproducer that mmap'ed a file backed by tmpfs with -o
huge=always, and then spawned 20 threads all looping faulting random
offsets in this map, while using madvise(MADV_DONTNEED) randomly for huge
page aligned ranges.  This very quickly reproduced the problem.

The problem here is that we check for the case that we have multiple
threads faulting in a range that was previously unmapped.  One thread maps
the PMD, the other thread loses the race and then returns 0.  However at
this point we already have the page, and we are no longer putting this
page into the processes address space, and so we leak the page.  We
actually did the correct thing prior to f9ce0be71d, however it looks
like Kirill copied what we do in the anonymous page case.  In the
anonymous page case we don't yet have a page, so we don't have to drop a
reference on anything.  Previously we did the correct thing for file based
faults by returning VM_FAULT_NOPAGE so we correctly drop the reference on
the page we faulted in.

Fix this by returning VM_FAULT_NOPAGE in the pmd_devmap_trans_unstable()
case, this makes us drop the ref on the page properly, and now my
reproducer no longer leaks the huge pages.

[josef@toxicpanda.com: v2]
  Link: https://lkml.kernel.org/r/e90c8f0dbae836632b669c2afc434006a00d4a67.1657721478.git.josef@toxicpanda.com
Link: https://lkml.kernel.org/r/2b798acfd95c9ab9395fe85e8d5a835e2e10a920.1657051137.git.josef@toxicpanda.com
Fixes: f9ce0be71d ("mm: Cleanup faultaround and finish_fault() codepaths")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Chris Mason <clm@fb.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2022-07-18 15:07:51 -07:00
arch riscv/mm: fix build error while PAGE_TABLE_CHECK enabled without MMU 2022-07-03 15:42:32 -07:00
block block: remove WARN_ON() from bd_link_disk_holder 2022-06-23 07:48:05 -06:00
certs Certs changes 2022-06-21 12:13:53 -05:00
crypto Certs changes 2022-06-21 12:13:53 -05:00
Documentation Minor things, mainly - mailmap updates, MAINTAINERS updates, etc. 2022-06-26 14:00:55 -07:00
drivers ARM: SoC fixes for 5.19 2022-06-26 14:12:56 -07:00
fs nilfs2: fix incorrect masking of permission flags for symlinks 2022-07-03 15:42:33 -07:00
include Documentation: highmem: use literal block for code example in highmem.h comment 2022-07-03 15:42:32 -07:00
init gcc-12: disable '-Warray-bounds' universally for now 2022-06-09 10:11:12 -07:00
ipc These changes update the ipc sysctls so that they are fundamentally 2022-06-03 15:54:57 -07:00
kernel Minor things, mainly - mailmap updates, MAINTAINERS updates, etc. 2022-06-26 14:00:55 -07:00
lib Build tool updates: 2022-06-19 09:54:16 -05:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm: fix page leak with multiple threads mapping the same page 2022-07-18 15:07:51 -07:00
net Networking fixes for 5.19-rc4, including fixes from bpf and netfilter. 2022-06-23 09:01:01 -05:00
samples fprobe, samples: Add use_trace option and show hit/missed counter 2022-06-17 21:53:29 +02:00
scripts kbuild: Ignore __this_module in gen_autoksyms.sh 2022-06-26 06:15:05 +09:00
security selinux: free contexts previously transferred in selinux_add_opt() 2022-06-15 21:20:45 -04:00
sound sound fixes for 5.19-rc4 2022-06-23 08:44:00 -05:00
tools tools headers UAPI: Synch KVM's svm.h header with the kernel 2022-06-26 12:32:55 -03:00
usr Not a lot of material this cycle. Many singleton patches against various 2022-05-27 11:22:03 -07:00
virt KVM: x86: disable preemption around the call to kvm_arch_vcpu_{un|}blocking 2022-06-09 10:52:20 -04:00
.clang-format clang-format: Fix space after for_each macros 2022-05-20 19:27:16 +02:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore kbuild: split the second line of *.mod into *.usyms 2022-05-08 03:16:59 +09:00
.mailmap mailmap: update Seth Forshee's email address 2022-07-18 15:07:51 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: replace a Microchip AT91 maintainer 2022-02-09 11:30:01 +01:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS ARM: SoC fixes for 5.19 2022-06-26 14:12:56 -07:00
Makefile Linux 5.19-rc4 2022-06-26 14:22:10 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

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.