linux-kernel - Re: Ack to merge through DRM? WAS Re: [PATCH v2 1/5] mm: Add write-protect and clean utilities for address space ranges

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Date:   Wed, 2 Oct 2019 15:28:10 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: Ack to merge through DRM? WAS Re: [PATCH v2 1/5] mm: Add
 write-protect and clean utilities for address space ranges

On 10/2/19 3:18 PM, Kirill A. Shutemov wrote:
> On Wed, Oct 02, 2019 at 11:21:01AM +0200, Thomas Hellström (VMware) wrote:
>> On 9/26/19 10:16 PM, Linus Torvalds wrote:
>>> On Thu, Sep 26, 2019 at 1:09 PM Thomas Hellström (VMware)
>>> <thomas_os@...pmail.org> wrote:
>>>> That said, if people are OK with me modifying the assert in
>>>> pud_trans_huge_lock() and make __walk_page_range non-static, it should
>>>> probably be possible to make it work, yes.
>>> I don't think you need to modify that assert at all.
>>>
>>> That thing only exists when there's a "pud_entry" op in the walker,
>>> and then you absolutely need to have that mmap_lock.
>>>
>>> As far as I can tell, you fundamentally only ever work on a pte level
>>> in your address space walker already and actually have a WARN_ON() on
>>> the pud_huge thing, so no pud entry can possibly apply.
>>>
>>> So no, the assert in pud_trans_huge_lock() does not seem to be a
>>> reason not to just use the existing page table walkers.
>>>
>>> And once you get rid of the walking, what is left? Just the "iterate
>>> over the inode mappings" part. Which could just be done in
>>> mm/pagewalk.c, and then you don't even need to remove the static.
>>>
>>> So making it be just another walking in pagewalk.c would seem to be
>>> the simplest model.
>>>
>>> Call it "walk_page_mapping()". And talk extensively about how the
>>> locking differs a lot from the usual "walk_page_vma()" things.
>>>
>>> The then actual "apply" functions (what a horrid name) could be in the
>>> users. They shouldn't be mixed in with the walking functions anyway.
>>> They are callbacks, not walkers.
>>>
>>>                Linus
>> Linus, Kirill
>>
>> I've pushed a reworked version based on the pagewalk code here:
>>
>> https://cgit.freedesktop.org/~thomash/linux/log/?h=pagewalk
>>
>> (top three patched)
>>
>> with users included here:
>>
>> https://cgit.freedesktop.org/~thomash/linux/log/?h=coherent-rebased
>>
>> Do you think this could work? The reason that the "mm: Add write-protect and
>> clean.." code is still in mm as a set of helpers, is of course that much of
>> the needed functionality is not exported, presumably since we want to keep
>> page table manipulation in mm.
> Could you post it to the mailing list? It's easier to review this way.
>
Sure.

/Thomas