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

[Mike Kravetz]

On 04/17/23 10:33, Michal Hocko wrote:
> On Fri 14-04-23 17:47:28, David Rientjes wrote:
> > On Thu, 13 Apr 2023, Michal Hocko wrote:
> > 
> > > [...]
> > > > > > 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.
> > > 
> > > I do not remember anybody hitting this with the current __GFP_NORETRY.
> > > So arguably there is nothing to be fixed ATM.
> > > 
> > 
> > I think we should still at least clear __GFP_NORETRY in this allocation: 
> > to be able to free 1GB hugepages back to the system we'd like the page 
> > allocator to at least exercise its normal order-0 allocation logic rather 
> > than exempting it from retrying reclaim by opting into __GFP_NORETRY.
> > 
> > I'd agree with the analysis in 
> > https://lore.kernel.org/linux-mm/YCafit5ruRJ+SL8I@xxxxxxxxxxxxxx/ that 
> > either a cleared __GFP_NORETRY or a __GFP_RETRY_MAYFAIL makes logical 
> > sense.
> > 
> > We really *do* want to free these hugepages back to the system and the 
> > amount of memory freeing will always be more than the allocation for 
> > struct page.  The net result is more free memory.
> > 
> > If the allocation fails, we can't free 1GB back to the system on a 
> > saturated node if our first reclaim attempt didn't allow these struct 
> > pages to be allocated.  Stranding 1GB in the hugetlb pool that no 
> > userspace on the system can make use of at the time isn't very useful.
> 
> I do not think there is any dispute in the theoretical concern. The question is
> whether this is something that really needs a fix in practice. Have we
> ever seen workloads which rely on GB pages to fail freeing them?

Since I have never seen a failure allocating vmemmmap, I agree that this
is all a theoretical concern.

However, to me it seems that replacing __GFP_NORETRY with __GFP_RETRY_MAYFAIL
would lessen that theoretical concern just a little.  That is simply because
an allocation with __GFP_RETRY_MAYFAIL would be a little more likely to
succeed.

Again, I know this is all theoretical but if switching to __GFP_RETRY_MAYFAIL
would prevent one allocation/hugetlb page freeing failure I think it is worth
it.  Because, as soon as we see one failure we may need to look into addressing
this now theoretical concern.
-- 
Mike Kravetz