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Peter Xu fe4cdc2c4e mm/userfaultfd: fix release hang over concurrent GUP 
This patch should fix a possible userfaultfd release() hang during
concurrent GUP.

This problem was initially reported by Dimitris Siakavaras in July 2023
[1] in a firecracker use case.  Firecracker has a separate process
handling page faults remotely, and when the process releases the
userfaultfd it can race with a concurrent GUP from KVM trying to fault in
a guest page during the secondary MMU page fault process.

A similar problem was reported recently again by Jinjiang Tu in March 2025
[2], even though the race happened this time with a mlockall() operation,
which does GUP in a similar fashion.

In 2017, commit 656710a60e ("userfaultfd: non-cooperative: closing the
uffd without triggering SIGBUS") was trying to fix this issue.  AFAIU,
that fixes well the fault paths but may not work yet for GUP.  In GUP, the
issue is NOPAGE will be almost treated the same as "page fault resolved"
in faultin_page(), then the GUP will follow page again, seeing page
missing, and it'll keep going into a live lock situation as reported.

This change makes core mm return RETRY instead of NOPAGE for both the GUP
and fault paths, proactively releasing the mmap read lock.  This should
guarantee the other release thread make progress on taking the write lock
and avoid the live lock even for GUP.

When at it, rearrange the comments to make sure it's uptodate.

[1] https://lore.kernel.org/r/79375b71-db2e-3e66-346b-254c90d915e2@cslab.ece.ntua.gr
[2] https://lore.kernel.org/r/20250307072133.3522652-1-tujinjiang@huawei.com

Link: https://lkml.kernel.org/r/20250312145131.1143062-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Jinjiang Tu <tujinjiang@huawei.com>
Cc: Dimitris Siakavaras <jimsiak@cslab.ece.ntua.gr>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2025-04-01 15:14:42 -07:00
arch Driver core updates for 6.15-rc1 2025-04-01 11:02:03 -07:00
block for-6.15/block-20250322 2025-03-26 18:08:55 -07:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto This update includes the following changes: 2025-03-29 10:01:55 -07:00
Documentation Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
drivers Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
fs mm/userfaultfd: fix release hang over concurrent GUP 2025-04-01 15:14:42 -07:00
include Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
init [ Merge note: this pull request depends on you having merged 2025-03-24 22:06:11 -07:00
io_uring for-6.15/io_uring-reg-vec-20250327 2025-03-28 15:07:04 -07:00
ipc treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
kernel Driver core updates for 6.15-rc1 2025-04-01 11:02:03 -07:00
lib Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm - The 6 patch series "Enable strict percpu address space checks" from 2025-04-01 09:29:18 -07:00
net - The 6 patch series "Enable strict percpu address space checks" from 2025-04-01 09:29:18 -07:00
rust Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
samples Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
scripts Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
security Driver core updates for 6.15-rc1 2025-04-01 11:02:03 -07:00
sound - The 7 patch series "powerpc/crash: use generic crashkernel 2025-04-01 10:06:52 -07:00
tools Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
usr kbuild: hdrcheck: fix cross build with clang 2025-03-05 04:06:45 +09:00
virt ARM: 2025-03-25 14:22:07 -07:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: give Clippy the minimum supported Rust version 2025-01-10 00:17:25 +01:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: Retire Ralf Baechle 2024-11-12 15:48:59 +01:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: use host dylib naming convention to support macOS 2025-01-10 01:01:24 +01:00
.mailmap - The 7 patch series "powerpc/crash: use generic crashkernel 2025-04-01 10:06:52 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS - The 6 patch series "Enable strict percpu address space checks" from 2025-04-01 09:29:18 -07:00
Kbuild drm: ensure drm headers are self-contained and pass kernel-doc 2025-02-12 10:44:43 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS Char/Misc/IIO driver updates for 6.15-rc1 2025-04-01 11:26:08 -07:00
Makefile [ Merge note: this pull request depends on you having merged 2025-03-24 22:06:11 -07:00
README README: Fix spelling 2024-03-18 03:36:32 -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 reStructuredText 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.