The patch titled
Subject: mm/gup: don't implicitly set FOLL_HONOR_NUMA_FAULT
has been added to the -mm mm-unstable branch. Its filename is
mm-gup-dont-implicitly-set-foll_honor_numa_fault.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-gup-dont-implicitly-set-foll_honor_numa_fault.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/gup: don't implicitly set FOLL_HONOR_NUMA_FAULT
Date: Thu, 3 Aug 2023 16:32:05 +0200
Commit 0b9d705297b2 ("mm: numa: Support NUMA hinting page faults from
gup/gup_fast") from 2012 documented as the primary reason why we would want
to handle NUMA hinting faults from GUP:
KVM secondary MMU page faults will trigger the NUMA hinting page
faults through gup_fast -> get_user_pages -> follow_page ->
handle_mm_fault.
That is still the case today, and relevant KVM code has been converted to
manually set FOLL_HONOR_NUMA_FAULT. So let's stop setting
FOLL_HONOR_NUMA_FAULT for all GUP users and cross fingers that not that
many other ones that really require such handling for autonuma remain.
Possible interaction with MMU notifiers:
Assume a driver obtains a page using get_user_pages() to map it into
a secondary MMU, and uses the MMU notifier framework to get notified on
changes.
Assume get_user_pages() succeeded on a PROT_NONE-mapped page (because
FOLL_HONOR_NUMA_FAULT is not set) in an accessible VMA and the page is
mapped into a secondary MMU. Once user space would turn that mapping
inaccessible using mprotect(PROT_NONE), the actual PTE in the page table
might not change. If the MMU notifier would be smart and optimize for that
case "why notify if the PTE didn't change", that could be problematic.
At least change_pmd_range() with MMU_NOTIFY_PROTECTION_VMA for now does an
unconditional mmu_notifier_invalidate_range_start() ->
mmu_notifier_invalidate_range_end() and should be fine.
Note that even if a PTE in an accessible VMA is pte_protnone(), the
underlying page might be accessed by a secondary MMU that does not set
FOLL_HONOR_NUMA_FAULT, and test_young() MMU notifiers would return "true".
Link: https://lkml.kernel.org/r/20230803143208.383663-5-david@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: liubo <liubo254@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/gup.c | 7 -------
1 file changed, 7 deletions(-)
--- a/mm/gup.c~mm-gup-dont-implicitly-set-foll_honor_numa_fault
+++ a/mm/gup.c
@@ -2244,13 +2244,6 @@ static bool is_valid_gup_args(struct pag
gup_flags |= FOLL_UNLOCKABLE;
}
- /*
- * For now, always trigger NUMA hinting faults. Some GUP users like
- * KVM require the hint to be as the calling context of GUP is
- * functionally similar to a memory reference from task context.
- */
- gup_flags |= FOLL_HONOR_NUMA_FAULT;
-
/* FOLL_GET and FOLL_PIN are mutually exclusive. */
if (WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) ==
(FOLL_PIN | FOLL_GET)))
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-gup-reintroduce-foll_numa-as-foll_honor_numa_fault.patch
smaps-use-vm_normal_page_pmd-instead-of-follow_trans_huge_pmd.patch
mm-memory_hotplug-document-the-signal_pending-check-in-offline_pages.patch
kvm-explicitly-set-foll_honor_numa_fault-in-hva_to_pfn_slow.patch
mm-gup-dont-implicitly-set-foll_honor_numa_fault.patch
pgtable-improve-pte_protnone-comment.patch
selftest-mm-ksm_functional_tests-test-in-mmap_and_merge_range-if-anything-got-merged.patch
selftest-mm-ksm_functional_tests-add-prot_none-test.patch
