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

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Date:   Thu, 15 Jun 2017 07:06:16 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will.deacon@....com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vineet Gupta <vgupta@...opsys.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "David S. Miller" <davem@...emloft.net>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, mark.rutland@....com
Subject: Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages



On Wednesday 14 June 2017 10:30 PM, Vlastimil Babka wrote:
> On 06/14/2017 06:55 PM, Will Deacon wrote:
>>>
>>> May be we should relook at pmd PTE udpate interface. We really need an
>>> interface that can update pmd entries such that we don't clear it in
>>> between. IMHO, we can avoid the pmdp_invalidate() completely, if we can
>>> switch from a pmd PTE entry to a pointer to PTE page (pgtable_t). We also
>>> need this interface to avoid the madvise race fixed by
>>
>> There's a good chance I'm not following your suggestion here, but it's
>> probably worth me pointing out that swizzling a page table entry from a
>> block mapping (e.g. a huge page mapped at the PMD level) to a table entry
>> (e.g. a pointer to a page of PTEs) can lead to all sorts of horrible
>> problems on ARM, including amalgamation of TLB entries and fatal aborts.
> 
> AFAIK some AMD x86_64 CPU's had the same problem and generated MCE's,
> and on Intel there are some restrictions when you can do that. See the
> large comment in __split_huge_pmd_locked().
> 

I was wondering whether we can do pmdp_establish(pgtable); and document 
all quirks needed for that in the per arch implementation of 
pmdp_establish(). We could also then switch all the 
pmdp_clear/set_pmd_at() usage to pmdp_establish().

-aneesh