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

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Date:   Wed, 12 Apr 2023 13:13:02 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mike.kravetz@...cle.com, mhocko@...e.com, muchun.song@...ux.dev,
        rientjes@...gle.com, souravpanda@...gle.com
Subject: Re: [PATCH v2] mm: hugetlb_vmemmap: provide stronger vmemmap
 allocation guarantees

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)?