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

[Pasha Tatashin]

On Thu, Apr 13, 2023 at 11:25 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 13-04-23 11:05:20, Pavel Tatashin wrote:
> > 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.
>
> This is really an important "detail" changelog should mention. While I
> am not really against that change I would much rather see that as a
> result of a real world fix rather than a theoretical concern. Mostly
> because a real life scenario would allow us to test the
> __GFP_RETRY_MAYFAIL effectivness. As that request might fail as well we
> just end up with a theoretical fix for a theoretical problem. Something
> that is easy to introduce but much harder to get rid of should we ever
> need to change __GFP_RETRY_MAYFAIL implementation for example.

I will add this to changelog in v3. If  __GFP_RETRY_MAYFAIL is
ineffective we will receive feedback once someone hits this problem.
Otherwise, we will never hear about it. I think overall it is safer to
keep this code with __GFP_RETRY_MAYFAIL flag.

>
> > 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.
>
> Yes, this really bothered me when the concept was introduced initially.
> I am always concerned when you need to allocate in order to free memory.
> Practically speaking we haven't heard about bug reports so maybe this is
> not such a big deal as I thought.

I suspect this is because at the moment it is not that frequent when a
machine is reconfigured from having a lot of HugeTLB based workload to
non-HugeTLB workload.

Pasha