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

Andrew Morton
2022-09-11T23:48:36Z

On Thu, 11 Aug 2022 12:13:29 -0400 Peter Xu <peterx@redhat.com> 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?

Thanks.