linux-kernel - Re: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages

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Date:   Wed, 29 Aug 2018 20:39:06 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared
 pages

On Wed 29-08-18 14:14:25, Jerome Glisse wrote:
> On Wed, Aug 29, 2018 at 10:24:44AM -0700, Mike Kravetz wrote:
[...]
> > What would be the best mmu notifier interface to use where there are no
> > start/end calls?
> > Or, is the best solution to add the start/end calls as is done in later
> > versions of the code?  If that is the suggestion, has there been any change
> > in invalidate start/end semantics that we should take into account?
> 
> start/end would be the one to add, 4.4 seems broken in respect to THP
> and mmu notification. Another solution is to fix user of mmu notifier,
> they were only a handful back then. For instance properly adjust the
> address to match first address covered by pmd or pud and passing down
> correct page size to mmu_notifier_invalidate_page() would allow to fix
> this easily.
> 
> This is ok because user of try_to_unmap_one() replace the pte/pmd/pud
> with an invalid one (either poison, migration or swap) inside the
> function. So anyone racing would synchronize on those special entry
> hence why it is fine to delay mmu_notifier_invalidate_page() to after
> dropping the page table lock.
> 
> Adding start/end might the solution with less code churn as you would
> only need to change try_to_unmap_one().

What about dependencies? 369ea8242c0fb sounds like it needs work for all
notifiers need to be updated as well.

Anyway, I am wondering why we haven't see any bugs coming from
incomplete range invalidation. How would those exhibit?
-- 
Michal Hocko
SUSE Labs