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 11:21:01 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     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 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.

Thanks,

Thomas