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

[Muchun Song]

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.

> --
> Michal Hocko
> SUSE Labs