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

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Message-ID: <d589ad0a-d5d4-927a-597c-4b094285d4b1@suse.cz>
Date:   Wed, 14 Jun 2017 19:00:47 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Will Deacon <will.deacon@....com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.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 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().

> So we really need to go via an invalid entry, with appropriate TLB
> invalidation before installing the new entry.
> 
> Will
>