Subject: Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Will Deacon <will.deacon@arm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, Vineet Gupta <vgupta@synopsys.com>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        "David S. Miller" <davem@davemloft.net>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Andrea Arcangeli <aarcange@redhat.com>, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org, mark.rutland@arm.com
References: <20170614135143.25068-1-kirill.shutemov@linux.intel.com>
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 <20170614165513.GD17632@arm.com>
 <548e33cb-e737-bb39-91a3-f66ee9211262@linux.vnet.ibm.com>
 <20170615084851.if6sntxo5tswhlk5@node.shutemov.name>
From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Date: Thu, 15 Jun 2017 15:06:49 +0530
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On Thursday 15 June 2017 02:18 PM, Kirill A. Shutemov wrote:
> O
>> I am not suggesting we don't do the invalidate (the need for that is
>> documented in __split_huge_pmd_locked(). I am suggesting we need a new
>> interface, something like Andrea suggested.
>>
>> old_pmd = pmdp_establish(pmd_mknotpresent());
>>
>> instead of pmdp_invalidate(). We can then use this in scenarios where we
>> want to update pmd PTE entries, where right now we go through a pmdp_clear
>> and set_pmd path. We should really not do that for THP entries.
> 
> Which cases are you talking about? When do we need to clear pmd and set
> later?
> 

With the latest upstream I am finding the usage when we mark pte clean 
page_mkclean_one . Also there is a similar usage in 
migrate_misplaced_transhuge_page(). I haven't really verified whether 
they do cause any race. But my suggestion is, we should avoid the usage 
of set_pmd_at() unless we are creating a new pmd PTE entry. If we can 
provide pmdp_establish() we can achieve that easily.

-aneesh