* + mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch added to mm-unstable branch
@ 2022-10-18 0:46 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2022-10-18 0:46 UTC (permalink / raw)
To: mm-commits, willy, vbabka, shuah, peterx, jhubbard, jgg, hughd,
aarcange, david, akpm
The patch titled
Subject: mm/ksm: simplify break_ksm() to not rely on VM_FAULT_WRITE
has been added to the -mm mm-unstable branch. Its filename is
mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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 via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days
------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm/ksm: simplify break_ksm() to not rely on VM_FAULT_WRITE
Date: Fri, 30 Sep 2022 16:19:26 +0200
Now that GUP no longer requires VM_FAULT_WRITE, break_ksm() is the sole
remaining user of VM_FAULT_WRITE. As we also want to stop triggering a
fake write fault and instead use FAULT_FLAG_UNSHARE -- similar to
GUP-triggered unsharing when taking a R/O pin on a shared anonymous page
(including KSM pages), let's stop relying on VM_FAULT_WRITE.
Let's rework break_ksm() to not rely on the return value of
handle_mm_fault() anymore to figure out whether COW-breaking was
successful. Simply perform another follow_page() lookup to verify the
result.
While this makes break_ksm() slightly less efficient, we can simplify
handle_mm_fault() a little and easily switch to FAULT_FLAG_UNSHARE without
introducing similar KSM-specific behavior for FAULT_FLAG_UNSHARE.
In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge
performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in
a performance degradation of ~4% -- 5% (old: ~5250 MiB/s, new: ~5010
MiB/s).
I don't think that we particularly care about that performance drop when
unmerging. If it ever turns out to be an actual performance issue, we can
think about a better alternative for FAULT_FLAG_UNSHARE -- let's just keep
it simple for now.
Link: https://lkml.kernel.org/r/20220930141931.174362-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/ksm.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)
--- a/mm/ksm.c~mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write
+++ a/mm/ksm.c
@@ -440,26 +440,27 @@ static int break_ksm(struct vm_area_stru
vm_fault_t ret = 0;
do {
+ bool ksm_page = false;
+
cond_resched();
page = follow_page(vma, addr,
FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
if (IS_ERR_OR_NULL(page))
break;
if (PageKsm(page))
- ret = handle_mm_fault(vma, addr,
- FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
- NULL);
- else
- ret = VM_FAULT_WRITE;
+ ksm_page = true;
put_page(page);
- } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
+
+ if (!ksm_page)
+ return 0;
+ ret = handle_mm_fault(vma, addr,
+ FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
+ NULL);
+ } while (!(ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
/*
- * We must loop because handle_mm_fault() may back out if there's
- * any difficulty e.g. if pte accessed bit gets updated concurrently.
- *
- * VM_FAULT_WRITE is what we have been hoping for: it indicates that
- * COW has been broken, even if the vma does not permit VM_WRITE;
- * but note that a concurrent fault might break PageKsm for us.
+ * We must loop until we no longer find a KSM page because
+ * handle_mm_fault() may back out if there's any difficulty e.g. if
+ * pte accessed bit gets updated concurrently.
*
* VM_FAULT_SIGBUS could occur if we race with truncation of the
* backing file, which also invalidates anonymous pages: that's
_
Patches currently in -mm which might be from david@redhat.com are
selftests-vm-add-test-to-measure-madv_unmergeable-performance.patch
mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
mm-remove-vm_fault_write.patch
mm-ksm-fix-ksm-cow-breaking-with-userfaultfd-wp-via-fault_flag_unshare.patch
mm-pagewalk-add-walk_page_range_vma.patch
mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch
mm-gup-remove-foll_migration.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
* + mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch added to mm-unstable branch
@ 2022-10-21 20:01 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2022-10-21 20:01 UTC (permalink / raw)
To: mm-commits, willy, vbabka, shuah, peterx, jhubbard, jgg, hughd,
aarcange, david, akpm
The patch titled
Subject: mm/ksm: simplify break_ksm() to not rely on VM_FAULT_WRITE
has been added to the -mm mm-unstable branch. Its filename is
mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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 via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days
------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm/ksm: simplify break_ksm() to not rely on VM_FAULT_WRITE
Date: Fri, 21 Oct 2022 12:11:34 +0200
Now that GUP no longer requires VM_FAULT_WRITE, break_ksm() is the sole
remaining user of VM_FAULT_WRITE. As we also want to stop triggering a
fake write fault and instead use FAULT_FLAG_UNSHARE -- similar to
GUP-triggered unsharing when taking a R/O pin on a shared anonymous page
(including KSM pages), let's stop relying on VM_FAULT_WRITE.
Let's rework break_ksm() to not rely on the return value of
handle_mm_fault() anymore to figure out whether COW-breaking was
successful. Simply perform another follow_page() lookup to verify the
result.
While this makes break_ksm() slightly less efficient, we can simplify
handle_mm_fault() a little and easily switch to FAULT_FLAG_UNSHARE without
introducing similar KSM-specific behavior for FAULT_FLAG_UNSHARE.
In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge
performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in
a performance degradation of ~4% -- 5% (old: ~5250 MiB/s, new: ~5010
MiB/s).
I don't think that we particularly care about that performance drop when
unmerging. If it ever turns out to be an actual performance issue, we can
think about a better alternative for FAULT_FLAG_UNSHARE -- let's just keep
it simple for now.
Link: https://lkml.kernel.org/r/20221021101141.84170-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/ksm.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)
--- a/mm/ksm.c~mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write
+++ a/mm/ksm.c
@@ -440,26 +440,27 @@ static int break_ksm(struct vm_area_stru
vm_fault_t ret = 0;
do {
+ bool ksm_page = false;
+
cond_resched();
page = follow_page(vma, addr,
FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
if (IS_ERR_OR_NULL(page))
break;
if (PageKsm(page))
- ret = handle_mm_fault(vma, addr,
- FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
- NULL);
- else
- ret = VM_FAULT_WRITE;
+ ksm_page = true;
put_page(page);
- } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
+
+ if (!ksm_page)
+ return 0;
+ ret = handle_mm_fault(vma, addr,
+ FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
+ NULL);
+ } while (!(ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
/*
- * We must loop because handle_mm_fault() may back out if there's
- * any difficulty e.g. if pte accessed bit gets updated concurrently.
- *
- * VM_FAULT_WRITE is what we have been hoping for: it indicates that
- * COW has been broken, even if the vma does not permit VM_WRITE;
- * but note that a concurrent fault might break PageKsm for us.
+ * We must loop until we no longer find a KSM page because
+ * handle_mm_fault() may back out if there's any difficulty e.g. if
+ * pte accessed bit gets updated concurrently.
*
* VM_FAULT_SIGBUS could occur if we race with truncation of the
* backing file, which also invalidates anonymous pages: that's
_
Patches currently in -mm which might be from david@redhat.com are
selftests-vm-anon_cow-test-cow-handling-of-anonymous-memory.patch
selftests-vm-factor-out-pagemap_is_populated-into-vm_util.patch
selftests-vm-anon_cow-thp-tests.patch
selftests-vm-anon_cow-hugetlb-tests.patch
selftests-vm-anon_cow-add-liburing-test-cases.patch
mm-gup_test-start-stop-read-functionality-for-pin-longterm-test.patch
mm-gup_test-start-stop-read-functionality-for-pin-longterm-test-fix.patch
selftests-vm-anon_cow-add-r-o-longterm-tests-via-gup_test.patch
selftests-vm-add-ksm-unmerge-tests.patch
mm-pagewalk-dont-trigger-test_walk-in-walk_page_vma.patch
selftests-vm-add-test-to-measure-madv_unmergeable-performance.patch
mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
mm-remove-vm_fault_write.patch
mm-ksm-fix-ksm-cow-breaking-with-userfaultfd-wp-via-fault_flag_unshare.patch
mm-pagewalk-add-walk_page_range_vma.patch
mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch
mm-gup-remove-foll_migration.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2022-10-21 20:02 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2022-10-21 20:01 + mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch added to mm-unstable branch Andrew Morton
-- strict thread matches above, loose matches on Subject: below --
2022-10-18 0:46 Andrew Morton
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.