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

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Date:   Sat, 22 Aug 2020 18:12:58 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        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>,
        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 12:18:41PM -0700, Linus Torvalds wrote:
> 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.

Yeah, it is certainly not the nicest thing to have in generic mm code,
but at least it is an improvement of the vmalloc_sync_all() interface we
had before, where the function had to be called at random undefined
places.

And x86-32 needs it, as long as we have the !SHARED_KERNEL_PMD cases
(which includes legacy paging). Or we also pre-allocate the PMDs on
x86-32 and forbid large ioremap mappings. But since the vmalloc area
gets larger with less RAM on x86-32, this would penalize low memory
machines by using more pages for the pre-allocations.

Not sure if making the vmalloc area on x86-32 a fixed 128MB range of
address space independent of RAM size is doable or if it will break some
machines. But with that pre-allocating PMDs would make more sense and we
could get rid of the p?d_alloc_track() stuff.

Regards,

	Joerg