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SubjectRe: [PATCH RFC 0/4] mm: Remember young bit for migration entries
Peter Xu <> writes:

> [Marking as RFC; only x86 is supported for now, plan to add a few more
> archs when there's a formal version]
> Problem
> =======
> When migrate a page, right now we always mark the migrated page as old.
> The reason could be that we don't really know whether the page is hot or
> cold, so we could have taken it a default negative assuming that's safer.
> 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 (as long as the MMU supports).
> 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 bit in the
> migration entries and carry it 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 young, and that's how this series tried to approach this
> problem.
> One tricky thing here is even though we're embedding the information into
> swap entry which seems to be a very generic data structure, the number of
> bits that are free is still arch dependent. Not only because the size of
> swp_entry_t differs, but also due to the different layouts of swap ptes on
> different archs.

If my understanding were correct, max_swapfile_size() provides a
mechanism to identify the available bits with swp_entry_t and swap PTE
considered. We may take advantage of that?

And according to commit 377eeaa8e11f ("x86/speculation/l1tf: Limit swap
file size to MAX_PA/2"), the highest bit of swap offset needs to be 0 if
L1TF mitigation is enabled.

Cced Andi for confirmation.

Best Regards,
Huang, Ying

> Here, this series requires specific arch to define an extra macro called
> __ARCH_SWP_OFFSET_BITS represents the size of swp offset. With this
> information, the swap logic can know whether there's extra bits to use,
> then it'll remember the young bits when possible. By default, it'll keep
> the old behavior of keeping all migrated pages cold.
> 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.
> Currently __ARCH_SWP_OFFSET_BITS is only defined on x86 for this series and
> only tested on x86_64 with Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz.
> Patch Layout
> ============
> Patch 1: Add swp_offset_pfn() and apply to all pfn swap entries, we should
> also stop treating swp_offset() as PFN anymore because it can
> contain more information starting from next patch.
> Patch 2: The core patch to remember young bit in swap offsets.
> Patch 3: A cleanup for x86 32 bits pgtable.h.
> Patch 4: Define __ARCH_SWP_OFFSET_BITS on x86, enable young bit for migration
> Please review, thanks.
> [1]
> Peter Xu (4):
> mm/swap: Add swp_offset_pfn() to fetch PFN from swap entry
> mm: Remember young bit for page migrations
> mm/x86: Use SWP_TYPE_BITS in 3-level swap macros
> mm/x86: Define __ARCH_SWP_OFFSET_BITS
> arch/arm64/mm/hugetlbpage.c | 2 +-
> arch/x86/include/asm/pgtable-2level.h | 6 ++
> arch/x86/include/asm/pgtable-3level.h | 15 +++--
> arch/x86/include/asm/pgtable_64.h | 5 ++
> include/linux/swapops.h | 85 +++++++++++++++++++++++++--
> mm/hmm.c | 2 +-
> mm/huge_memory.c | 10 +++-
> mm/memory-failure.c | 2 +-
> mm/migrate.c | 4 +-
> mm/migrate_device.c | 2 +
> mm/page_vma_mapped.c | 6 +-
> mm/rmap.c | 3 +-
> 12 files changed, 122 insertions(+), 20 deletions(-)

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