The patch titled
Subject: mm: document locking restrictions for vm_operations_struct::close
has been added to the -mm tree. Its filename is
mm-document-locking-restrictions-for-vm_operations_struct-close.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-document-locking-restrictions-for-vm_operations_struct-close.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-document-locking-restrictions-for-vm_operations_struct-close.patch
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next and is updated
there every 3-4 working days
------------------------------------------------------
From: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Subject: mm: document locking restrictions for vm_operations_struct::close
Patch series "mm: protect free_pgtables with mmap_lock write lock in exit_mmap", v4.
oom-reaper and process_mrelease system call should protect against races
with exit_mmap which can destroy page tables while they walk the VMA tree.
oom-reaper protects from that race by setting MMF_OOM_VICTIM and by
relying on exit_mmap to set MMF_OOM_SKIP before taking and releasing
mmap_write_lock. process_mrelease has to elevate mm->mm_users to prevent
such race. Both oom-reaper and process_mrelease hold mmap_read_lock when
walking the VMA tree.
The locking rules and mechanisms could be simpler if exit_mmap takes
mmap_write_lock while executing destructive operations such as
free_pgtables.
Change exit_mmap to hold the mmap_write_lock when calling free_pgtables
and remove_vma. Operations like unmap_vmas and unlock_range are not
destructive and could run under mmap_read_lock but for simplicity we take
one mmap_write_lock during almost the entire operation. Note also that
because oom-reaper checks VM_LOCKED flag, unlock_range() should not be
allowed to race with it.
Before this patch, remove_vma used to be called with no locks held,
however with fput being executed asynchronously and vm_ops->close not
being allowed to hold mmap_lock (it is called from __split_vma with
mmap_sem held for write), changing that should be fine.
In most cases this lock should be uncontended. Previously, Kirill
reported ~4% regression caused by a similar change [1]. We reran the same
test and although the individual results are quite noisy, the percentiles
show lower regression with 1.6% being the worst case [2]. The change
allows oom-reaper and process_mrelease to execute safely under
mmap_read_lock without worries that exit_mmap might destroy page tables
from under them.
[1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@xxxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@xxxxxxxxxxxxxx/
This patch (of 2):
Add comments for vm_operations_struct::close documenting locking
requirements for this callback and its callers.
Link: https://lkml.kernel.org/r/20211208212211.2860249-1-surenb@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20211208212211.2860249-2-surenb@xxxxxxxxxx
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Roman Gushchin <guro@xxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Christian Brauner <christian@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
Cc: Florian Weimer <fweimer@xxxxxxxxxx>
Cc: Jan Engelhardt <jengelh@xxxxxxx>
Cc: Tim Murray <timmurray@xxxxxxxxxx>
Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 4 ++++
1 file changed, 4 insertions(+)
--- a/include/linux/mm.h~mm-document-locking-restrictions-for-vm_operations_struct-close
+++ a/include/linux/mm.h
@@ -532,6 +532,10 @@ enum page_entry_size {
*/
struct vm_operations_struct {
void (*open)(struct vm_area_struct * area);
+ /**
+ * @close: Called when the VMA is being removed from the MM.
+ * Context: User context. May sleep. Caller holds mmap_lock.
+ */
void (*close)(struct vm_area_struct * area);
/* Called any time before splitting to check if it's allowed */
int (*may_split)(struct vm_area_struct *area, unsigned long addr);
_
Patches currently in -mm which might be from surenb@xxxxxxxxxx are
mm-add-a-field-to-store-names-for-private-anonymous-memory-fix.patch
mm-add-anonymous-vma-name-refcounting.patch
mm-document-locking-restrictions-for-vm_operations_struct-close.patch
mm-oom_kill-allow-process_mrelease-to-run-under-mmap_lock-protection.patch
sysctl-change-watermark_scale_factor-max-limit-to-30%.patch
