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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Message-ID: <CAHk-=whwN+CvaorsmczZRwFhSV+1x98xg-zajVD1qKmN=9JhBQ@mail.gmail.com>
Date:   Thu, 26 Sep 2019 13:16:55 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
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 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