Re: PageLRU and the other flags

[Yang Shi <shy828301@gmail.com>]

On Fri, Sep 25, 2020 at 8:21 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 25.09.20 17:00, Matthew Wilcox wrote:
> > I'm not quite familiar with this side of the MM yet, but it seems to
> > me like we're encoding four page states in three bits:
> >
> >  - !PageLRU (not an LRU page)
> >  - PageLRU (on the inactive list)
> >  - PageLRU + PageActive (on the active list)
> >  - PageLRU + PageUnevictable (on the unevictable list)
> >
> > Can we neaten this up into two bits?
> >
> >  - 00 (not an LRU page)
> >  - 01 (active list)
> >  - 10 (inactive list)
> >  - 11 (unevictable)
> >
> > People who free up page flags are always popular, right?
>
> They are the best :)
>
> Looking at __pagevec_lru_add_fn()
>
> SetPageLRU(page);
> smp_mb__after_atomic();
> ...
>
> if (page_evictable(page)) {
>         ...
> } else {
>         lru = LRU_UNEVICTABLE;
>         ClearPageActive(page);
>         SetPageUnevictable(page);
>         ...
> }
>
> Looks like PageActive(page) could have been relevant even without
> SetPageLRU(page);
>
> So the combo
>
> !PageLRU() + PageActive (previously was on the active list maybe?)
>
> might be valid? Or dead code? Or valid races (someone else spotting
> PageLRU() and setting it PageActive(page))?

Yes, it is valid. The page might be marked active when it is on
pagevec, then will be moved to active LRU in the next drain. Please
refer to the comment in mark_page_accessed().

> But no expert, just blindly poking around ...
>
> --
> Thanks,
>
> David / dhildenb
>
>