linux-kernel - Re: [RFC PATCH 00/16] 1GB THP support on x86

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Date:   Wed, 2 Sep 2020 16:29:46 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
CC:     <linux-mm@...ck.org>, Roman Gushchin <guro@...com>,
        Rik van Riel <riel@...riel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Nellans <dnellans@...dia.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/16] 1GB THP support on x86_64

On 2 Sep 2020, at 15:57, Jason Gunthorpe wrote:

> On Wed, Sep 02, 2020 at 03:05:39PM -0400, Zi Yan wrote:
>> On 2 Sep 2020, at 14:48, Jason Gunthorpe wrote:
>>
>>> On Wed, Sep 02, 2020 at 02:45:37PM -0400, Zi Yan wrote:
>>>
>>>>> Surprised this doesn't touch mm/pagewalk.c ?
>>>>
>>>> 1GB PUD page support is present for DAX purpose, so the code is there
>>>> in mm/pagewalk.c already. I only needed to supply ops->pud_entry when using
>>>> the functions in mm/pagewalk.c. :)
>>>
>>> Yes, but doesn't this change what is possible under the mmap_sem
>>> without the page table locks?
>>>
>>> ie I would expect some thing like pmd_trans_unstable() to be required
>>> as well for lockless walkers. (and I don't think the pmd code is 100%
>>> right either)
>>>
>>
>> Right. I missed that. Thanks for pointing it out.
>> The code like this, right?
>
> Technically all those *pud's are racy too, the design here with the
> _unstable function call always seemed weird. I strongly suspect it
> should mirror how get_user_pages_fast works for lockless walking

You mean READ_ONCE on page table entry pointer first, then use the value
for the rest of the loop? I am not quite familiar with this racy check
part of the code and happy to hear more about it.


—
Best Regards,
Yan Zi

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