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 01:48:29, Muchun Song 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.

Then do a due research or ask for a help from the MM community. Do
not just try to go around harder problems and somehow duct tape a
solution. I am sorry for sounding harsh here but this is a repetitive
pattern.

Now to the merit. put_page can indeed be called from all sorts of
contexts. And it might be indeed impossible to guarantee that hugetlb
pages are never freed up from an atomic context. Requiring that would be
even hard to maintain longterm. There are ways around that, I believe,
though.

The most simple one that I can think of right now would be using
in_atomic() rather than in_task() check free_huge_page. IIRC recent
changes would allow in_atomic to be reliable also on !PREEMPT kernels
(via RCU tree, not sure where this stands right now). That would make
__free_huge_page always run in a non-atomic context which sounds like an
easy enough solution.
Another way would be to keep a pool of ready pages to use in case of
GFP_NOWAIT allocation fails and have means to keep that pool replenished
when needed. Would it be feasible to reused parts of the freed page in
the worst case?

-- 
Michal Hocko
SUSE Labs