Re: [PATCH 0/9] mm/mglru: suppress empty page table walks during aging

[Baoquan He <baoquan.he@linux.dev>]

On 08/28/26 at 02:11pm, Barry Song wrote:
> On Mon, Aug 24, 2026 at 3:38 PM Baoquan He <hebaoquan@kylinos.cn> wrote:
> >
> > In the current MGLRU, lru_gen_use_mm() will mark an mm for all nodes at
> > each context switch, so on each node aging will walk into each mm's page
> > table independently. On a multi-NUMA node system, one process launched
> > on one or a subset of nodes, its mm is walked by all other nodes's aging
> > while finds on pages for their lruvec. This is pure waste (100% walks
> 
> I assume you mean “finds no pages” rather than “find on pages.”

You are right, typo, thanks.

> 
> > are empty on those other nodes)
> >
> > This patch series suppresses these empty page table walks with two
> > complementary mechanisms at different levels:
> >
> >   - empty_map (cross-node specifc). At mm granularity, one process's mm
> >     whose walk found no pages for one node's lruvec is skipped on that
> >     node between re-scan passes.
> >
> >   - PUD-level Bloom filter (general). One level up from the existing PMD
> >     filters, it skips any 1GB PUD that had no young entries last
> >     generation - namely whose 512 PMDs all failed PMD test. Cross-node
> >     empty walks can prove its effect the best, but it also suppresses
> >     purely code PUD inside local mms. So it reduces unnecessary walking
> >     in any workload with cold areas.
> 
> I assume you mean “purely cold PUD” rather than “purely code PUD”.

Right, thanks.

> 
> >
> > The two complement each other: empty_map reduces the *number* of
> > cross-node walks, the PUD-level filter reduces the *cost* of the walks
> > that remain.
> 
> I wonder if the PUD-level filter alone would achieve your goal without
> requiring the more complex `empty_map` code.
> 
> PUD is already a fairly large granularity, so even if we still do some
> redundant work, I wonder if the overhead would be small enough to make
> `empty_map` unnecessary?

As replied to you in patch 2 thread, will take PUD filter alone in v2
as you suggested, thanks.