Re: [PATCH v2] mm: hugetlb_vmemmap: provide stronger vmemmap allocation guarantees

[Pasha Tatashin]

On Wed, Apr 12, 2023 at 4:18 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 12-04-23 13:13:02, Andrew Morton wrote:
> > Lots of questions (ie, missing information!)
> >
> > On Wed, 12 Apr 2023 19:59:39 +0000 Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:
> >
> > > HugeTLB pages have a struct page optimizations where struct pages for tail
> > > pages are freed. However, when HugeTLB pages are destroyed, the memory for
> > > struct pages (vmemmap) need to be allocated again.
> > >
> > > Currently, __GFP_NORETRY flag is used to allocate the memory for vmemmap,
> > > but given that this flag makes very little effort to actually reclaim
> > > memory the returning of huge pages back to the system can be problem.
> >
> > Are there any reports of this happening in the real world?
> >
> > > Lets
> > > use __GFP_RETRY_MAYFAIL instead. This flag is also performs graceful
> > > reclaim without causing ooms, but at least it may perform a few retries,
> > > and will fail only when there is genuinely little amount of unused memory
> > > in the system.
> >
> > If so, does this change help?
> >
> > If the allocation attempt fails, what are the consequences?
> >
> > What are the potential downsides to this change?  Why did we choose
> > __GFP_NORETRY in the first place?
> >
> > What happens if we try harder (eg, GFP_KERNEL)?
>
> Mike was generous enough to make me remember
> https://lore.kernel.org/linux-mm/YCafit5ruRJ+SL8I@xxxxxxxxxxxxxx/.
> GFP_KERNEL wouldn't make much difference becauset this is
> __GFP_THISNODE. But I do agree that the changelog should go into more
> details about why do we want to try harder now. I can imagine that
> shrinking hugetlb pool by a large amount of hugetlb pages might become a
> problem but is this really happening or is this a theoretical concern?

This is a theoretical concern. Freeing a 1G page requires 16M of free
memory. A machine might need to be reconfigured from one task to
another, and release a large number of 1G pages back to the system if
allocating 16M fails, the release won't work.

In an ideal scenario we should guarantee that this never fails: that
we always can free HugeTLB pages back to the system. At the very least
we could steal the memory for vmemmap from the page that is being
released.

Pasha