linux-kernel - Re: [PATCH] hugetlb: set hugetlb page flag before optimizing vmemmap

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Message-Id: <00753B00-69DB-499E-8BC3-5DF42DC5936B@linux.dev>
Date:   Wed, 30 Aug 2023 11:32:27 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Usama Arif <usama.arif@...edance.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        James Houghton <jthoughton@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlb: set hugetlb page flag before optimizing vmemmap



> On Aug 30, 2023, at 05:37, Mike Kravetz <mike.kravetz@...cle.com> wrote:
> 
> Currently, vmemmap optimization of hugetlb pages is performed before the
> hugetlb flag (previously hugetlb destructor) is set identifying it as a
> hugetlb folio.  This means there is a window of time where an ordinary
> folio does not have all associated vmemmap present.  The core mm only
> expects vmemmap to be potentially optimized for hugetlb  and device dax.
> This can cause problems in code such as memory error handling that may
> want to write to tail struct pages.
> 
> There is only one call to perform hugetlb vmemmap optimization today.
> To fix this issue, simply set the hugetlb flag before that call.
> 
> There was a similar issue in the free hugetlb path that was previously
> addressed.  The two routines that optimize or restore hugetlb vmemmap
> should only be passed hugetlb folios/pages.  To catch any callers not
> following this rule, add VM_WARN_ON calls to the routines.  In the
> hugetlb free code paths, some calls could be made to restore vmemmap
> after clearing the hugetlb flag.  This was 'safe' as in these cases
> vmemmap was already present and the call was a NOOP.  However, for
> consistency these calls where eliminated so that we can add the
> VM_WARN_ON checks.
> 
> Fixes: f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with each HugeTLB page")
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

Thanks for your fix.

Reviewed-by: Muchun Song <songmuchun@...edance.com>