Re: [PATCH] mm: Optimize TLB flushes during page reclaim

[Vinay Banakar <vny@google.com>]

On Wed, Jan 22, 2025 at 2:05 PM SeongJae Park <sj@kernel.org> wrote:
> damon_pa_pageout() from mm/damon/paddr.c also calls shrink_folio_list() similar
> to madvise.c, but it doesn't aware such batching behavior.  Have you checked
> that path?

Thanks for catching this path. In damon_pa_pageout(),
shrink_folio_list() processes all pages from a single NUMA node that
were collected (filtered) from a single DAMON region (r->ar.start to
r->ar.end). This means it could be processing anywhere from 1 page up
to ULONG_MAX pages from a single node at once. With the patch, we'll
send a single IPI for TLB flush for the entire region, reducing IPIs
by a factor equal to the number of pages being reclaimed by DAMON at
once (decided by damon_reclaim_quota).

My only concern here would be the overhead of maintaining the
temporary pageout_list for batching. However, during BIO submission,
the patch checks if the folio was reactivated, so submitting to BIO in
bulk should be safe.

Another option would be to modify shrink_folio_list() to force batch
flushes for up to N pages (512) at a time, rather than relying on
callers to do the batching via folio_list.

Thanks!
Vinay