Re: [PATCH v2] mm: Track page table modifications in __apply_to_page_range()

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Aug 21, 2020 at 5:38 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> From: Joerg Roedel <jroedel@suse.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