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

[Michal Hocko <mhocko@suse.com>]

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@dhcp22.suse.cz/ 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?

-- 
Michal Hocko
SUSE Labs