Re: [External] Re: [PATCH v15 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Michal Hocko]

On Tue 16-02-21 02:19:20, Muchun Song wrote:
> On Tue, Feb 16, 2021 at 1:48 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, Feb 16, 2021 at 12:28 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Mon 15-02-21 23:36:49, Muchun Song wrote:
> > > [...]
> > > > > There shouldn't be any real reason why the memory allocation for
> > > > > vmemmaps, or handling vmemmap in general, has to be done from within the
> > > > > hugetlb lock and therefore requiring a non-sleeping semantic. All that
> > > > > can be deferred to a more relaxed context. If you want to make a
> > > >
> > > > Yeah, you are right. We can put the freeing hugetlb routine to a
> > > > workqueue. Just like I do in the previous version (before v13) patch.
> > > > I will pick up these patches.
> > >
> > > I haven't seen your v13 and I will unlikely have time to revisit that
> > > version. I just wanted to point out that the actual allocation doesn't
> > > have to happen from under the spinlock. There are multiple ways to go
> > > around that. Dropping the lock would be one of them. Preallocation
> > > before the spin lock is taken is another. WQ is certainly an option but
> > > I would take it as the last resort when other paths are not feasible.
> > >
> >
> > "Dropping the lock" and "Preallocation before the spin lock" can limit
> > the context of put_page to non-atomic context. I am not sure if there
> > is a page puted somewhere under an atomic context. e.g. compaction.
> > I am not an expert on this.
> 
> Using GFP_KERNEL will also use the current task cpuset to allocate
> memory. Do we have an interface to ignore current task cpuset？If not,
> WQ may be the only option and it also will not limit the context of
> put_page. Right?

Well, GFP_KERNEL is constrained to the task cpuset only if the said
cpuset is hardwalled IIRC. But I do not see why this is a problem.
-- 
Michal Hocko
SUSE Labs