linux/arch/powerpc
Peter Xu 4064b98270 mm: allow VM_FAULT_RETRY for multiple times
The idea comes from a discussion between Linus and Andrea [1].

Before this patch we only allow a page fault to retry once.  We achieved
this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing
handle_mm_fault() the second time.  This was majorly used to avoid
unexpected starvation of the system by looping over forever to handle the
page fault on a single page.  However that should hardly happen, and after
all for each code path to return a VM_FAULT_RETRY we'll first wait for a
condition (during which time we should possibly yield the cpu) to happen
before VM_FAULT_RETRY is really returned.

This patch removes the restriction by keeping the FAULT_FLAG_ALLOW_RETRY
flag when we receive VM_FAULT_RETRY.  It means that the page fault handler
now can retry the page fault for multiple times if necessary without the
need to generate another page fault event.  Meanwhile we still keep the
FAULT_FLAG_TRIED flag so page fault handler can still identify whether a
page fault is the first attempt or not.

Then we'll have these combinations of fault flags (only considering
ALLOW_RETRY flag and TRIED flag):

  - ALLOW_RETRY and !TRIED:  this means the page fault allows to
                             retry, and this is the first try

  - ALLOW_RETRY and TRIED:   this means the page fault allows to
                             retry, and this is not the first try

  - !ALLOW_RETRY and !TRIED: this means the page fault does not allow
                             to retry at all

  - !ALLOW_RETRY and TRIED:  this is forbidden and should never be used

In existing code we have multiple places that has taken special care of
the first condition above by checking against (fault_flags &
FAULT_FLAG_ALLOW_RETRY).  This patch introduces a simple helper to detect
the first retry of a page fault by checking against both (fault_flags &
FAULT_FLAG_ALLOW_RETRY) and !(fault_flag & FAULT_FLAG_TRIED) because now
even the 2nd try will have the ALLOW_RETRY set, then use that helper in
all existing special paths.  One example is in __lock_page_or_retry(), now
we'll drop the mmap_sem only in the first attempt of page fault and we'll
keep it in follow up retries, so old locking behavior will be retained.

This will be a nice enhancement for current code [2] at the same time a
supporting material for the future userfaultfd-writeprotect work, since in
that work there will always be an explicit userfault writeprotect retry
for protected pages, and if that cannot resolve the page fault (e.g., when
userfaultfd-writeprotect is used in conjunction with swapped pages) then
we'll possibly need a 3rd retry of the page fault.  It might also benefit
other potential users who will have similar requirement like userfault
write-protection.

GUP code is not touched yet and will be covered in follow up patch.

Please read the thread below for more information.

[1] https://lore.kernel.org/lkml/20171102193644.GB22686@redhat.com/
[2] https://lore.kernel.org/lkml/20181230154648.GB9832@redhat.com/

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Brian Geffon <bgeffon@google.com>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Martin Cracauer <cracauer@cons.org>
Cc: Marty McFadden <mcfadden8@llnl.gov>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Maya Gokhale <gokhale2@llnl.gov>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Link: http://lkml.kernel.org/r/20200220160246.9790-1-peterx@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2020-04-02 09:35:30 -07:00
..
boot Kbuild updates for v5.6 (2nd) 2020-02-09 16:05:50 -08:00
configs virtio: fixes, cleanups 2020-02-07 12:26:34 -08:00
crypto crypto: remove CRYPTO_TFM_RES_BAD_KEY_LEN 2020-01-09 11:30:53 +08:00
include asm-generic: make more kernel-space headers mandatory 2020-04-02 09:35:25 -07:00
kernel bpf: Support llvm-objcopy for vmlinux BTF 2020-03-19 12:32:38 +01:00
kexec powerpc: Replace cpu_up/down() with add/remove_cpu() 2020-03-25 12:59:35 +01:00
kvm KVM: PPC: Fix kernel crash with PR KVM 2020-03-20 13:39:10 +11:00
lib powerpc: Fix __clear_user() with KUAP enabled 2019-12-16 23:19:44 +11:00
math-emu
mm mm: allow VM_FAULT_RETRY for multiple times 2020-04-02 09:35:30 -07:00
net treewide: Use sizeof_field() macro 2019-12-09 10:36:44 -08:00
oprofile powerpc updates for 5.6 2020-02-04 13:06:46 +00:00
perf perf/core: Add new branch sample type for HW index of raw branch records 2020-02-11 13:23:49 +01:00
platforms powerpc/ps3: Convert half completion to rcuwait 2020-03-21 16:00:22 +01:00
purgatory
sysdev powerpc updates for 5.6 2020-02-04 13:06:46 +00:00
tools powerpc: Do not consider weak unresolved symbol relocations as bad 2020-01-31 20:17:22 +11:00
xmon powerpc/xmon: Fix whitespace handling in getstring() 2020-02-18 21:31:12 +11:00
Kbuild powerpc/kexec: Move kexec files into a dedicated subdir. 2019-11-21 15:41:34 +11:00
Kconfig context-tracking: Introduce CONFIG_HAVE_TIF_NOHZ 2020-02-14 16:05:04 +01:00
Kconfig.debug powerpc/ptdump: Only enable PPC_CHECK_WX with STRICT_KERNEL_RWX 2020-01-23 21:31:13 +11:00
Makefile powerpc: Don't add -mabi= flags when building with Clang 2019-11-25 21:45:43 +11:00
Makefile.postlink powerpc: Do not consider weak unresolved symbol relocations as bad 2020-01-31 20:17:22 +11:00