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

From: Andrew Morton
Date: Sun Sep 11 2022 - 19:48:36 EST

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?