linux-kernel - Re: [PATCH v2] mm: Track page table modifications in __apply_to_page

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Date:   Fri, 21 Aug 2020 12:18:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        Pavel Machek <pavel@....cz>, Dave Airlie <airlied@...hat.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Vrabel <david.vrabel@...rix.com>,
        Joerg Roedel <jroedel@...e.de>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2] mm: Track page table modifications in __apply_to_page_range()

On Fri, Aug 21, 2020 at 5:38 AM Joerg Roedel <joro@...tes.org> wrote:
>
> From: Joerg Roedel <jroedel@...e.de>
>
> The __apply_to_page_range() function is also used to change and/or
> allocate page-table pages in the vmalloc area of the address space.
> Make sure these changes get synchronized to other page-tables in the
> system by calling arch_sync_kernel_mappings() when necessary.

I get the strong feeling that these functions should be using a
"struct apply_details *" or something like that (the way the
zap_page_range() code has that "zap_details" thing).

Because adding more and more arguments gets pretty painful after a
while. But maybe the compiler inlining it all makes it a non-issue.

It also strikes me that I think the only architecture that uses the
whole arch_sync_kernel_mappings() thing is now just x86-32.

[ Well, x86-64 still has it, but that's because we undid the 64-bit
removal, but it's on the verge of going away and x86-64 shouldn't
actually _need_ it any more ]

So all of this seems to be purely for 32-bit x86. Which kind of makes
this all fail the smell test.

But the patch does seem to be the minimal fix for a real issue - I'm
just pointing out ugly details, not actual problems with the patch.

IOW, a somewhat reluctant Ack, hoping that this will be cleaned up
some day. Possibly/hopefully because arch_sync_kernel_mappings() just
goes away entirely.

                 Linus