Re: [PATCH v3 5/8] mm/migrate: make a standard migration target allocation function

[Joonsoo Kim <js1304@gmail.com>]

2020년 6월 26일 (금) 오후 4:33, Michal Hocko <mhocko@kernel.org>님이 작성:
>
> On Fri 26-06-20 14:02:49, Joonsoo Kim wrote:
> > 2020년 6월 25일 (목) 오후 9:05, Michal Hocko <mhocko@kernel.org>님이 작성:
> > >
> > > On Tue 23-06-20 15:13:45, Joonsoo Kim wrote:
> [...]
> > > > -struct page *new_page_nodemask(struct page *page,
> > > > -                             int preferred_nid, nodemask_t *nodemask)
> > > > +struct page *alloc_migration_target(struct page *page, unsigned long private)
> > > >  {
> > > > -     gfp_t gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL;
> > > > +     struct migration_target_control *mtc;
> > > > +     gfp_t gfp_mask;
> > > >       unsigned int order = 0;
> > > >       struct page *new_page = NULL;
> > > > +     int zidx;
> > > > +
> > > > +     mtc = (struct migration_target_control *)private;
> > > > +     gfp_mask = mtc->gfp_mask;
> > > >
> > > >       if (PageHuge(page)) {
> > > >               return alloc_huge_page_nodemask(
> > > > -                             page_hstate(compound_head(page)),
> > > > -                             preferred_nid, nodemask, 0, false);
> > > > +                             page_hstate(compound_head(page)), mtc->nid,
> > > > +                             mtc->nmask, gfp_mask, false);
> > > >       }
> > > >
> > > >       if (PageTransHuge(page)) {
> > > > +             gfp_mask &= ~__GFP_RECLAIM;
> > >
> > > What's up with this gfp_mask modification?
> >
> > THP page allocation uses a standard gfp masks, GFP_TRANSHUGE_LIGHT and
> > GFP_TRANHUGE. __GFP_RECLAIM flags is a big part of this standard mask design.
> > So, I clear it here so as not to disrupt the THP gfp mask.
>
> Why this wasn't really needed before? I guess I must be missing
> something here. This patch should be mostly mechanical convergence of
> existing migration callbacks but this change adds a new behavior AFAICS.

Before this patch, a user cannot specify a gfp_mask and THP allocation
uses GFP_TRANSHUGE
statically. After this patch, a user can specify a gfp_mask and it
could conflict with GFP_TRANSHUGE.
This code tries to avoid this conflict.

> It would effectively drop __GFP_RETRY_MAYFAIL and __GFP_KSWAPD_RECLAIM

__GFP_RETRY_MAYFAIL isn't dropped. __GFP_RECLAIM is
"___GFP_DIRECT_RECLAIM|___GFP_KSWAPD_RECLAIM".
So, __GFP_KSWAPD_RECLAIM would be dropped for THP allocation.
IIUC, THP allocation doesn't use __GFP_KSWAPD_RECLAIM since it's overhead is
too large and this overhead should be given to the caller rather than
system thread (kswapd)
and so on.

Thanks.