Re: [PATCH v7 04/12] mm: multigenerational LRU: groundwork

[Yu Zhao <yuzhao@google.com>]

On Thu, Feb 10, 2022 at 09:37:25PM +0000, Matthew Wilcox wrote:
> On Tue, Feb 08, 2022 at 01:18:54AM -0700, Yu Zhao wrote:

[...]

> > +++ b/include/linux/page-flags-layout.h
> > @@ -26,6 +26,14 @@
> >  
> >  #define ZONES_WIDTH		ZONES_SHIFT
> >  
> > +#ifdef CONFIG_LRU_GEN
> > +/* LRU_GEN_WIDTH is generated from order_base_2(CONFIG_NR_LRU_GENS + 1). */
> > +#define LRU_REFS_WIDTH		(CONFIG_TIERS_PER_GEN - 2)
> > +#else
> > +#define LRU_GEN_WIDTH		0
> > +#define LRU_REFS_WIDTH		0
> > +#endif /* CONFIG_LRU_GEN */
> 
> I'm concerned about the number of bits being used in page->flags.
> It seems to me that we already have six bits in use to aid us in choosing
> which pages to reclaim: referenced, lru, active, workingset, reclaim,
> unevictable.
> 
> What I was hoping to see from this patch set was reuse of those bits.

Agreed. I have a plan to *reduce* some of those bits but it's a
relatively low priority item on my to-do list.

> That would give us 32 queues in total.  Some would be special (eg pages
> cannot migrate out of the unevictable queue), but it seems to me that you
> effectively have 4 queues for active and 4 queues for inactive at this
> point (unless I misunderstood that).  I think we need special numbers
> for: Not on the LRU and Unevictable, but that still leaves us with 30
> generations to split between active & inactive.
> 
> But maybe we still need some of those bits?  Perhaps it's not OK to say
> that queue id 0 is !LRU, queue 1 is unevictable, queue #2 is workingset,
> queues 3-7 are active, queues 8-15 are various degrees of inactive.
> I'm assuming that it's not sensible to have a page that's marked as both
> "reclaim" and "workingset", but perhaps it is.
> 
> Anyway, I don't understand this area well enough.  I was just hoping
> for some simplification.

I plan to use the spare bits in folio->lru to indicate which lru list
a folio is on, i.e., active/inactive or generations or unevictable.

In addition, swapbacked could go to folio->mapping -- we wouldn't need
it if there were no MADV_FREE, i.e., it would be equivalent to
PageAnon() || shmem_mapping().

These two work items can be done separately and in parallel with
everything else that's been going on lately. It'd awesome if somebody
volunteers, and I can find some resource from our side to test/review
the code so that we can have them done sooner.

The rest, in theory, can also be moved to somewhere but, IMO, it's not
really worth the effort given the situation isn't dire at the moment.
referenced and workingset are already reused between the active/inactive
lru and the multigenerational lru; reclaim is reused for readahead, but
readahead could be split out as an xa tag; lru is reused for isolation
synchronization.