Re: [PATCH] mm/rmap: always do TTU_IGNORE_ACCESS

[Johannes Weiner]

On Wed, Nov 04, 2020 at 03:19:28PM -0800, Shakeel Butt wrote:
> Since the commit 369ea8242c0f ("mm/rmap: update to new mmu_notifier
> semantic v2"), the code to check the secondary MMU's page table access
> bit is broken for !(TTU_IGNORE_ACCESS) because the page is unmapped from
> the secondary MMU's page table before the check. More specifically for
> those secondary MMUs which unmap the memory in
> mmu_notifier_invalidate_range_start() like kvm.
> 
> However memory reclaim is the only user of !(TTU_IGNORE_ACCESS) or the
> absence of TTU_IGNORE_ACCESS and it explicitly performs the page table
> access check before trying to unmap the page. So, at worst the reclaim
> will miss accesses in a very short window if we remove page table access
> check in unmapping code.

We also miss accesses that happen right after the unmap :-) Seems
completely fine to make page_referenced() the time of last call.

> There is an unintented consequence of !(TTU_IGNORE_ACCESS) for the memcg
> reclaim. From memcg reclaim the page_referenced() only account the
> accesses from the processes which are in the same memcg of the target
> page but the unmapping code is considering accesses from all the
> processes, so, decreasing the effectiveness of memcg reclaim.
> 
> The simplest solution is to always assume TTU_IGNORE_ACCESS in unmapping
> code.
> 
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>