linux-kernel - Re: [PATCH 0/2] Fix hugetlb free path race with memory errors

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Date:   Thu, 13 Jul 2023 10:34:07 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jiaqi Yan <jiaqiyan@...gle.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        James Houghton <jthoughton@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 0/2] Fix hugetlb free path race with memory errors

On Tue, 11 Jul 2023 15:09:40 -0700 Mike Kravetz <mike.kravetz@...cle.com> wrote:

> In the discussion of Jiaqi Yan's series "Improve hugetlbfs read on
> HWPOISON hugepages" the race window was discovered.
> https://lore.kernel.org/linux-mm/20230616233447.GB7371@monkey/
> 
> Freeing a hugetlb page back to low level memory allocators is performed
> in two steps.
> 1) Under hugetlb lock, remove page from hugetlb lists and clear destructor
> 2) Outside lock, allocate vmemmap if necessary and call low level free
> Between these two steps, the hugetlb page will appear as a normal
> compound page.  However, vmemmap for tail pages could be missing.
> If a memory error occurs at this time, we could try to update page
> flags non-existant page structs.
> 
> A much more detailed description is in the first patch.
> 
> The first patch addresses the race window.  However, it adds a
> hugetlb_lock lock/unlock cycle to every vmemmap optimized hugetlb
> page free operation.  This could lead to slowdowns if one is freeing
> a large number of hugetlb pages.
> 
> The second path optimizes the update_and_free_pages_bulk routine
> to only take the lock once in bulk operations.
> 
> The second patch is technically not a bug fix, but includes a Fixes
> tag and Cc stable to avoid a performance regression.  It can be
> combined with the first, but was done separately make reviewing easier.
> 

I feel that backporting performance improvements into -stable is not a
usual thing to do.  Perhaps the fact that it's a regression fix changes
this, but why?

Much hinges on the magnitude of the performance change.  Are you able
to quantify this at all?