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authorPeter Xu <peterx@redhat.com>2022-08-11 18:13:25 +0200
committerAndrew Morton <akpm@linux-foundation.org>2022-09-27 04:46:04 +0200
commit9c61d5321e94a4d0678b5eb0515afc590bdb9740 (patch)
tree8b431f5597633c96834f433d04d0987bb8e0a3b8
parentmm, hwpoison: cleanup some obsolete comments (diff)
downloadlinux-9c61d5321e94a4d0678b5eb0515afc590bdb9740.tar.xz
linux-9c61d5321e94a4d0678b5eb0515afc590bdb9740.zip
mm/x86: use SWP_TYPE_BITS in 3-level swap macros
Patch series "mm: Remember a/d bits for migration entries", v4. Problem ======= When migrating a page, right now we always mark the migrated page as old & clean. However that could lead to at least two problems: (1) We lost the real hot/cold information while we could have persisted. That information shouldn't change even if the backing page is changed after the migration, (2) There can be always extra overhead on the immediate next access to any migrated page, because hardware MMU needs cycles to set the young bit again for reads, and dirty bits for write, as long as the hardware MMU supports these bits. Many of the recent upstream works showed that (2) is not something trivial and actually very measurable. In my test case, reading 1G chunk of memory - jumping in page size intervals - could take 99ms just because of the extra setting on the young bit on a generic x86_64 system, comparing to 4ms if young set. This issue is originally reported by Andrea Arcangeli. Solution ======== To solve this problem, this patchset tries to remember the young/dirty bits in the migration entries and carry them over when recovering the ptes. We have the chance to do so because in many systems the swap offset is not really fully used. Migration entries use swp offset to store PFN only, while the PFN is normally not as large as swp offset and normally smaller. It means we do have some free bits in swp offset that we can use to store things like A/D bits, and that's how this series tried to approach this problem. max_swapfile_size() is used here to detect per-arch offset length in swp entries. We'll automatically remember the A/D bits when we find that we have enough swp offset field to keep both the PFN and the extra bits. Since max_swapfile_size() can be slow, the last two patches cache the results for it and also swap_migration_ad_supported as a whole. Known Issues / TODOs ==================== We still haven't taught madvise() to recognize the new A/D bits in migration entries, namely MADV_COLD/MADV_FREE. E.g. when MADV_COLD upon a migration entry. It's not clear yet on whether we should clear the A bit, or we should just drop the entry directly. We didn't teach idle page tracking on the new migration entries, because it'll need larger rework on the tree on rmap pgtable walk. However it should make it already better because before this patchset page will be old page after migration, so the series will fix potential false negative of idle page tracking when pages were migrated before observing. The other thing is migration A/D bits will not start to working for private device swap entries. The code is there for completeness but since private device swap entries do not yet have fields to store A/D bits, even if we'll persistent A/D across present pte switching to migration entry, we'll lose it again when the migration entry converted to private device swap entry. Tests ===== After the patchset applied, the immediate read access test [1] of above 1G chunk after migration can shrink from 99ms to 4ms. The test is done by moving 1G pages from node 0->1->0 then read it in page size jumps. The test is with Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz. Similar effect can also be measured when writting the memory the 1st time after migration. After applying the patchset, both initial immediate read/write after page migrated will perform similarly like before migration happened. Patch Layout ============ Patch 1-2: Cleanups from either previous versions or on swapops.h macros. Patch 3-4: Prepare for the introduction of migration A/D bits Patch 5: The core patch to remember young/dirty bit in swap offsets. Patch 6-7: Cache relevant fields to make migration_entry_supports_ad() fast. [1] https://github.com/xzpeter/clibs/blob/master/misc/swap-young.c This patch (of 7): Replace all the magic "5" with the macro. Link: https://lkml.kernel.org/r/20220811161331.37055-1-peterx@redhat.com Link: https://lkml.kernel.org/r/20220811161331.37055-2-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: David Hildenbrand <david@redhat.com> Reviewed-by: Huang Ying <ying.huang@intel.com> Cc: Hugh Dickins <hughd@google.com> Cc: "Kirill A . Shutemov" <kirill@shutemov.name> Cc: Alistair Popple <apopple@nvidia.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Andi Kleen <andi.kleen@intel.com> Cc: Nadav Amit <nadav.amit@gmail.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Dave Hansen <dave.hansen@intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
-rw-r--r--arch/x86/include/asm/pgtable-3level.h8
1 files changed, 4 insertions, 4 deletions
diff --git a/arch/x86/include/asm/pgtable-3level.h b/arch/x86/include/asm/pgtable-3level.h
index e896ebef8c24..28421a887209 100644
--- a/arch/x86/include/asm/pgtable-3level.h
+++ b/arch/x86/include/asm/pgtable-3level.h
@@ -256,10 +256,10 @@ static inline pud_t native_pudp_get_and_clear(pud_t *pudp)
/* We always extract/encode the offset by shifting it all the way up, and then down again */
#define SWP_OFFSET_SHIFT (SWP_OFFSET_FIRST_BIT + SWP_TYPE_BITS)
-#define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > 5)
-#define __swp_type(x) (((x).val) & 0x1f)
-#define __swp_offset(x) ((x).val >> 5)
-#define __swp_entry(type, offset) ((swp_entry_t){(type) | (offset) << 5})
+#define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > SWP_TYPE_BITS)
+#define __swp_type(x) (((x).val) & ((1UL << SWP_TYPE_BITS) - 1))
+#define __swp_offset(x) ((x).val >> SWP_TYPE_BITS)
+#define __swp_entry(type, offset) ((swp_entry_t){(type) | (offset) << SWP_TYPE_BITS})
/*
* Normally, __swp_entry() converts from arch-independent swp_entry_t to