linux-kernel - Re: [PATCHv2 1/3] x86/mm: Provide pmdp

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Date:   Wed, 21 Jun 2017 10:20:47 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     Catalin Marinas <catalin.marinas@....com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "David S. Miller" <davem@...emloft.net>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper

On 06/21/2017 10:16 AM, Kirill A. Shutemov wrote:
> On Wed, Jun 21, 2017 at 08:49:03AM -0700, Vineet Gupta wrote:
>> On 06/21/2017 04:27 AM, Catalin Marinas wrote:
>>> On Wed, Jun 21, 2017 at 12:53:03PM +0300, Kirill A. Shutemov wrote:
>>>>>>>>> On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote:
>>>>>>>>>> We need an atomic way to setup pmd page table entry, avoiding races with
>>>>>>>>>> CPU setting dirty/accessed bits. This is required to implement
>>>>>>>>>> pmdp_invalidate() that doesn't loose these bits.
>>> [...]
>>>> Any chance you could help me with arm too?
>>> On arm (ARMv7 with LPAE) we don't have hardware updates of the
>>> access/dirty bits, so a generic implementation would suffice. I didn't
>>> find one in your patches, so here's an untested version:
>>>
>>> static inline pmd_t pmdp_establish(struct mm_struct *mm, unsigned long address,
>>> 				   pmd_t *pmdp, pmd_t pmd)
>>> {
>>> 	pmd_t old_pmd = *pmdp;
>>> 	set_pmd_at(mm, address, pmdp, pmd);
>>> 	return old_pmd;
>>> }
>> So it seems the discussions have settled down and pmdp_establish() can be
>> implemented in generic way as above and it will suffice if arch doesn't have
>> a special need. It would be nice to add the comment above generic version
>> that it only needs to be implemented if hardware sets the accessed/dirty
>> bits !
>>
>> Then nothing special is needed for ARC - right ?
> I will define generic version as Catalin proposed with a comment, but
> under the name generic_pmdp_establish. An arch can make use of it by
>
> #define pmdp_establish generic_pmdp_establish

Can you do that for ARC in your next posting - or want me to once you have posted 
that ?


> I don't want it to be used by default without attention from architecture
> maintainer. It can lead unnoticied breakage if THP got enabled on new
> arch.

Makes sense !

-Vineet