linux-kernel - Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages

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Date:   Wed, 14 Jun 2017 16:06:36 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vineet Gupta <vgupta@...opsys.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "David S. Miller" <davem@...emloft.net>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages

Hi Kirill,

On Wed, 14 Jun 2017 16:51:40 +0300
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> Vlastimil noted that pmdp_invalidate() is not atomic and we can loose
> dirty and access bits if CPU sets them after pmdp dereference, but
> before set_pmd_at().
> 
> The bug doesn't lead to user-visible misbehaviour in current kernel, but
> fixing this would be critical for future work on THP: both huge-ext4 and THP
> swap out rely on proper dirty tracking.
> 
> Unfortunately, there's no way to address the issue in a generic way. We need to
> fix all architectures that support THP one-by-one.
> 
> All architectures that have THP supported have to provide atomic
> pmdp_invalidate(). If generic implementation of pmdp_invalidate() is used,
> architecture needs to provide atomic pmdp_mknonpresent().
> 
> I've fixed the issue for x86, but I need help with the rest.
> 
> So far THP is supported on 8 architectures. Power and S390 already provides
> atomic pmdp_invalidate(). x86 is fixed by this patches, so 5 architectures
> left:

For s390 the pmdp_invalidate() is atomic only in regard to the dirty and
referenced bits because we use a fault driven approach for this, no?

More specifically the update via the pmdp_xchg_direct() function is protected
by the page table lock, the update on the pmd entry itself does *not* have
to be atomic (for s390).

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.