linux-kernel - Re: [PATCH v2 2/5] hugetlb: convert page_huge

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Date:   Thu, 21 Jan 2021 09:18:16 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/5] hugetlb: convert page_huge_active()
 HPageMigratable flag

On Wed, Jan 20, 2021 at 01:48:05PM -0800, Mike Kravetz wrote:
> >> This comment addresses both this patch and the next one.
> >>
> >> Instead of putting the SetHPageMigratable flag spread over the
> >> allocation paths, would it make more sense to place it in
> >> alloc_huge_page before returning the page?
> >> Then we could opencode SetHPageMigratableIfSupported right there.
> > 
> > and in putback_active_hugepage.
> 
> 
> Hi Oscar,
> 
> In Muchun's series of hugetlb bug fixes, Michal asked the same question.
> 
> https://lore.kernel.org/linux-mm/7e69a55c-d501-6b42-8225-a677f09fb829@oracle.com/
> 
> The 'short answer' is that the this would allow a page to be migrated
> after allocation but before the page fault code adds it to the page
> cache or page tables.  This actually caused bugs in the past.

Oh, I see. I jumped late into that patchset so I missed some early messages.
Thanks for explaining this again.

-- 
Oscar Salvador
SUSE L3