Re: [PATCH 1/2] mm: add vma_has_locality()

[Yu Zhao]

On Thu, Dec 22, 2022 at 11:49 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 21 Dec 2022 22:13:40 -0800 Yuanchu Xie <yuanchu@xxxxxxxxxx> wrote:
>
> > From: Yu Zhao <yuzhao@xxxxxxxxxx>

This works; suggested-by probably works even better, since I didn't do
the follow-up work.

> > Currently in vm_flags in vm_area_struct, both VM_SEQ_READ and
> > VM_RAND_READ indicate a lack of locality in accesses to the vma. Some
> > places that check for locality are missing one of them. We add
> > vma_has_locality to replace the existing locality checks for clarity.
>
> I'm all confused.  Surely VM_SEQ_READ implies locality and VM_RAND_READ
> indicates no-locality?

Spatially, yes. But we focus more on the temporal criteria here, i.e.,
the reuse of an area within a relatively small duration. Both the
active/inactive LRU and MGLRU rely on this.

VM_SEQ_READ, while being a special case of spatial locality, fails the
temporal criteria. VM_RAND_READ fails both criterias, obviously.

Once an area passes the temporal criteria, MGLRU additionally exploits
spatial locality by lru_gen_look_around(), which is also touched in
this patch. This part is good to know but not really relevant here.