Re: [RFC][PATCH 5/9] mm/migrate: demote pages during reclaim

[Yang Shi <shy828301@gmail.com>]

On Tue, Oct 27, 2020 at 8:29 AM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Wed, Oct 07, 2020 at 09:17:45AM -0700, Dave Hansen wrote:
> > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Yang Shi <yang.shi@linux.alibaba.com>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Huang Ying <ying.huang@intel.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
>
> I am still going through all the details, but just my thoughts on things
> that caught my eye:
>
> > --- a/include/linux/migrate.h~demote-with-migrate_pages       2020-10-07 09:15:31.028642442 -0700
> > +++ b/include/linux/migrate.h 2020-10-07 09:15:31.034642442 -0700
> > @@ -27,6 +27,7 @@ enum migrate_reason {
> >       MR_MEMPOLICY_MBIND,
> >       MR_NUMA_MISPLACED,
> >       MR_CONTIG_RANGE,
> > +     MR_DEMOTION,
> >       MR_TYPES
>
> I think you also need to add it under include/trace/events/migrate.h, so
> mm_migrate_pages event can know about it.

Agree.

>
> > +bool migrate_demote_page_ok(struct page *page, struct scan_control *sc)
>
> Make it static?
> Also, scan_control seems to be unused here.
>
> > +{
> > +     int next_nid = next_demotion_node(page_to_nid(page));
> > +
> > +     VM_BUG_ON_PAGE(!PageLocked(page), page);
>
> Right after the call to migrate_demote_page_ok, we call unlock_page
> which already has this check in place.
> I know that this is only to be on the safe side and we do not loss anything,
> but just my thoughts.
>
> > +static struct page *alloc_demote_page(struct page *page, unsigned long node)
> > +{
> > +     /*
> > +      * Try to fail quickly if memory on the target node is not
> > +      * available.  Leaving out __GFP_IO and __GFP_FS helps with
> > +      * this.  If the desintation node is full, we want kswapd to
> > +      * run there so that its pages will get reclaimed and future
> > +      * migration attempts may succeed.
> > +      */
> > +     gfp_t flags = (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_NORETRY |
> > +                    __GFP_NOMEMALLOC | __GFP_NOWARN | __GFP_THISNODE |
> > +                    __GFP_KSWAPD_RECLAIM);
>
> I think it would be nicer to have this as a real GFP_ thingy defined.
> e.g: GFP_DEMOTION
>
> > +     /* HugeTLB pages should not be on the LRU */
> > +     WARN_ON_ONCE(PageHuge(page));
>
> I am not sure about this one.
> This could only happen if the page, which now it is in another list, ends up in
> the buddy system. That is quite unlikely bth.
> And nevertheless, this is only a warning, which means that if this scenario gets
> to happen, we will be allocating a single page to satisfy a higher-order page, and
> I am not sure about the situation we will end up with.

IMHO, we should use BUG_ON instead of WARN_ON or we should just back
off if we see hugetlb page in this path and print out some warning.

>
> > +
> > +     if (PageTransHuge(page)) {
> > +             struct page *thp;
> > +
> > +             flags |= __GFP_COMP;
> > +
> > +             thp = alloc_pages_node(node, flags, HPAGE_PMD_ORDER);
> > +             if (!thp)
> > +                     return NULL;
> > +             prep_transhuge_page(thp);
> > +             return thp;
> > +     }
> > +
> > +     return __alloc_pages_node(node, flags, 0);
>
> Would make sense to transform this in some sort of new_demotion_page,
> which actually calls alloc_migration_target with the right stuff in place?
> And then pass a struct migration_target_control so alloc_migration_target
> does the right thing.
> alloc_migration_target also takes care of calling prep_transhuge_page
> when needed.
> e.g:
>
> static struct page *new_demotion_node(struct page *page, unsigned long private)
> {
>         struct migration_target_control mtc = {
>                 .nid = private,
>                 .gfp_mask = GFP_DEMOTION,
>         };
>
>         if (PageTransHuge(page))
>                 mtc.gfp_mask |= __GFP_COMP;
>
>         return alloc_migration_target(page, (unsigned long)&mtc);
> }
>
> The only thing I see is that alloc_migration_target seems to "override"
> the gfp_mask and does ORs GFP_TRANSHUGE for THP pages, which includes
> __GFP_DIRECT_RECLAIM (not appreciated in this case).
> But maybe this can be worked around by checking if gfp_mask == GFP_DEMOTION,
> and if so, just keep the mask as it is.

Makes sense to me.

>
> > +
> > +     if (list_empty(demote_pages))
> > +             return 0;
> > +
> > +     /* Demotion ignores all cpuset and mempolicy settings */
> > +     err = migrate_pages(demote_pages, alloc_demote_page, NULL,
> > +                         target_nid, MIGRATE_ASYNC, MR_DEMOTION,
> > +                         &nr_succeeded);
>
> As I said, instead of alloc_demote_page, use a new_demote_page and make
> alloc_migration_target handle the allocations and prep thp pages.
>
>
> --
> Oscar Salvador
> SUSE L3
>