linux-kernel - Re: [PATCH RESEND 0/8] hugetlb: add demote/split page functionality

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Date:   Fri, 27 Aug 2021 19:22:52 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>, Zi Yan <ziy@...dia.com>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH RESEND 0/8] hugetlb: add demote/split page functionality

On 8/25/21 00:08, Mike Kravetz wrote:
> Add Vlastimil and Hillf,
> 
> Well, I set up a test environment on a larger system to get some
> numbers.  My 'load' on the system was filling the page cache with
> clean pages.  The thought is that these pages could easily be reclaimed.
> 
> When trying to get numbers I hit a hugetlb page allocation stall where
> __alloc_pages(__GFP_RETRY_MAYFAIL, order 9) would stall forever (or at
> least an hour).  It was very much like the symptoms addressed here:
> https://lore.kernel.org/linux-mm/20190806014744.15446-1-mike.kravetz@oracle.com/
> 
> This was on 5.14.0-rc6-next-20210820.
> 
> I'll do some more digging as this appears to be some dark corner case of
> reclaim and/or compaction.  The 'good news' is that I can reproduce
> this.

Interesting, let's see if that's some kind of new regression.

>> And the second problem would benefit from some words to help us
>> understand how much real-world hurt this causes, and how frequently.
>> And let's understand what the userspace workarounds look like, etc.
> 
> The stall above was from doing a simple 'free 1GB page' followed by
> 'allocate 512 MB pages' from userspace.

Is the allocation different in any way than the usual hugepage allocation
possible today?

> Getting out another version of this series will be delayed, as I think
> we need to address or understand this issue first.
>