Re: [PATCH v3 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range

Lorenzo Stoakes (Oracle)
2026-04-07T09:39:55Z

On Sat, Apr 04, 2026 at 09:44:14PM -0700, Hugh Dickins wrote:
> Perhaps there is, or could be, a cleverer way for KSM to walk the anon_vma
> interval tree, which can handle the mremap-moved pgoffs appropriately.
> Cc'ing Michel, whose bf181b9f9d8d ("mm anon rmap: replace same_anon_vma
> linked list with an interval tree.") specifically chose the 0, ULONG_MAX
> which you are replacing.

No, I don't think there could be, and I wouldn't want anybody to try to
implement any kind of remap-tracking that might clash with my future work, not
that I think there's a hugely sensible way of doing that with the current
anon_vma implementation.

>
> Cc'ing Lorenzo, who is currently considering replacing anon_vma by
> something more like my anonmm, which preceded Andrea's anon_vma in 2.6.7;
> but Lorenzo supplementing it with the mremap tracking which defeated me.
> This rmap_walk_ksm() might well benefit from his approach. (I'm not
> actually expecting any input from Lorenzo here, or Michel: more FYIs.)

Thanks :)

Maybe I should go read your anonmm implementation... the mremap-tracking is
tricky but I have it working (modulo, KSM, yeah this whole thing was a good hint
that I need to look at that, too [+ whatever else I've missed so far]).

Bandwith is low for foreseeable future so expectations of not-reply are probably
fairly valid atm (and yet here I am, replying :)

>
> But more realistic in the short term, might be for you to keep your
> optimization, but fix the lookup, by keeping a count of PTEs found,
> and when that falls short, take a second pass with 0, ULONG_MAX.
> Somewhat ugly, certainly imperfect, but good enough for now.

Yeah that could work, it's not likely that many of these would be mremap()'d
right?

Yes ugly, but anon_vma is (very) ugly.

Cheers, Lorenzo