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

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Message-ID: <ZDcSG2t3/sVuZc67@dhcp22.suse.cz>
Date:   Wed, 12 Apr 2023 22:18:35 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Pasha Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mike.kravetz@...cle.com, muchun.song@...ux.dev,
        rientjes@...gle.com, souravpanda@...gle.com
Subject: Re: [PATCH v2] mm: hugetlb_vmemmap: provide stronger vmemmap
 allocation guarantees

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@...een.com> 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@dhcp22.suse.cz/.
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?

-- 
Michal Hocko
SUSE Labs