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Linus Torvalds f3c64eda3e mm: avoid early COW write protect games during fork() 
In commit 70e806e4e6 ("mm: Do early cow for pinned pages during fork()
for ptes") we write-protected the PTE before doing the page pinning
check, in order to avoid a race with concurrent fast-GUP pinning (which
doesn't take the mm semaphore or the page table lock).

That trick doesn't actually work - it doesn't handle memory ordering
properly, and doing so would be prohibitively expensive.

It also isn't really needed.  While we're moving in the direction of
allowing and supporting page pinning without marking the pinned area
with MADV_DONTFORK, the fact is that we've never really supported this
kind of odd "concurrent fork() and page pinning", and doing the
serialization on a pte level is just wrong.

We can add serialization with a per-mm sequence counter, so we know how
to solve that race properly, but we'll do that at a more appropriate
time.  Right now this just removes the write protect games.

It also turns out that the write protect games actually break on Power,
as reported by Aneesh Kumar:

 "Architecture like ppc64 expects set_pte_at to be not used for updating
  a valid pte. This is further explained in commit 56eecdb912 ("mm:
  Use ptep/pmdp_set_numa() for updating _PAGE_NUMA bit")"

and the code triggered a warning there:

  WARNING: CPU: 0 PID: 30613 at arch/powerpc/mm/pgtable.c:185 set_pte_at+0x2a8/0x3a0 arch/powerpc/mm/pgtable.c:185
  Call Trace:
    copy_present_page mm/memory.c:857 [inline]
    copy_present_pte mm/memory.c:899 [inline]
    copy_pte_range mm/memory.c:1014 [inline]
    copy_pmd_range mm/memory.c:1092 [inline]
    copy_pud_range mm/memory.c:1127 [inline]
    copy_p4d_range mm/memory.c:1150 [inline]
    copy_page_range+0x1f6c/0x2cc0 mm/memory.c:1212
    dup_mmap kernel/fork.c:592 [inline]
    dup_mm+0x77c/0xab0 kernel/fork.c:1355
    copy_mm kernel/fork.c:1411 [inline]
    copy_process+0x1f00/0x2740 kernel/fork.c:2070
    _do_fork+0xc4/0x10b0 kernel/fork.c:2429

Link: https://lore.kernel.org/lkml/CAHk-=wiWr+gO0Ro4LvnJBMs90OiePNyrE3E+pJvc9PzdBShdmw@mail.gmail.com/
Link: https://lore.kernel.org/linuxppc-dev/20201008092541.398079-1-aneesh.kumar@linux.ibm.com/
Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Tested-by: Leon Romanovsky <leonro@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Kirill Shutemov <kirill@shutemov.name>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2020-10-08 10:11:32 -07:00
arch Fix a kernel panic in the AES crypto code caused by a BR tail call not 2020-10-06 12:09:29 -07:00
block block-5.9-2020-10-02 2020-10-02 14:34:52 -07:00
certs
crypto
Documentation Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-10-05 11:27:14 -07:00
drivers platform-drivers-x86 for v5.9-3 2020-10-06 12:00:52 -07:00
fs splice: teach splice pipe reading about empty pipe buffers 2020-10-06 10:27:22 -07:00
include drm fixes for 5.9 final 2020-10-06 11:05:44 -07:00
init
ipc
kernel usermodehelper: reset umask to default before executing user process 2020-10-06 10:31:52 -07:00
lib drm-misc-fixes for v5.9: 2020-10-06 12:38:28 +10:00
LICENSES
mm mm: avoid early COW write protect games during fork() 2020-10-08 10:11:32 -07:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-10-05 11:27:14 -07:00
samples
scripts scripts/spelling.txt: fix malformed entry 2020-10-03 11:28:12 -07:00
security
sound
tools Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-10-05 11:27:14 -07:00
usr
virt
.clang-format
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore
.mailmap
COPYING
CREDITS
Kbuild
Kconfig
MAINTAINERS drm fixes for 5.9 final 2020-10-06 11:05:44 -07:00
Makefile Linux 5.9-rc8 2020-10-04 16:04:34 -07:00
README

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.