Re: [PATCH v4 5/7] mm: Remember young/dirty bit for page migrations

From: Huang, Ying
Date: Mon Sep 12 2022 - 20:55:55 EST

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Thu, 11 Aug 2022 12:13:29 -0400 Peter Xu <peterx@xxxxxxxxxx> wrote:
>> When page migration happens, we always ignore the young/dirty bit settings
>> in the old pgtable, and marking the page as old in the new page table using
>> either pte_mkold() or pmd_mkold(), and keeping the pte clean.
>> That's fine from functional-wise, but that's not friendly to page reclaim
>> because the moving page can be actively accessed within the procedure. Not
>> to mention hardware setting the young bit can bring quite some overhead on
>> some systems, e.g. x86_64 needs a few hundreds nanoseconds to set the bit.
>> The same slowdown problem to dirty bits when the memory is first written
>> after page migration happened.
>> Actually we can easily remember the A/D bit configuration and recover the
>> information after the page is migrated. To achieve it, define a new set of
>> bits in the migration swap offset field to cache the A/D bits for old pte.
>> Then when removing/recovering the migration entry, we can recover the A/D
>> bits even if the page changed.
>> One thing to mention is that here we used max_swapfile_size() to detect how
>> many swp offset bits we have, and we'll only enable this feature if we know
>> the swp offset is big enough to store both the PFN value and the A/D bits.
>> Otherwise the A/D bits are dropped like before.
> There was some discussion over v3 of this patch, but none over v4.
> Can people please review this patch series so we can get moving with it?

Most discussions over v3 are for migrate_device.c code. There are some
bugs and they have been fixed by Alistair via [1].

This patch itself is good. Sorry for bothering.

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>


Best Regards,
Huang, Ying