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Subject[PATCH RFC 0/4] mm: Remember young bit for migration entries
Date
[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.

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] https://github.com/xzpeter/clibs/blob/master/misc/swap-young.c

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(-)

--
2.32.0

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