linux-kernel - Re: [PATCH] mm/vmalloc: track which page-table levels were modified

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Message-Id: <20200604140617.e340dd507ee68b0a05bd21cb@linux-foundation.org>
Date:   Thu, 4 Jun 2020 14:06:17 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joerg Roedel <jroedel@...e.de>, Guenter Roeck <linux@...ck-us.net>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/vmalloc: track which page-table levels were modified

On Thu, 4 Jun 2020 10:16:07 -0700 Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Jun 4, 2020 at 1:35 AM Joerg Roedel <jroedel@...e.de> wrote:
> >
> > I posted the fix for this already:
> >
> >         https://lore.kernel.org/lkml/20200604074446.23944-1-joro@8bytes.org/
> 
> Ugh.
> 
> I was going to apply this directly, but as I looked at the patch I
> just found it fairly illegible.
> 
> Is there some reason why the 5level-fixup.h versions use that
> very-hard-to-follow macro, rather than the inline functions that the
> main mm.h file uses?
> 
> I'm _assuming_ it's because it gets included in some place where not
> everything is defined yet, so making it a macro means that it works
> (later on) when everything has come together..
> 
> But the solution to that would seem to make all the p.._alloc_track()
> macros just be in a different header file, and make them be all
> together. We already have that
> 
>    #if !__ARCH_HAS_5LEVEL_HACK
> 
> in linux/mm.h, so it's not like we really have isolated that issue
> into just 5level-fixup.h anyway, and creating a new
> <linux/pagetable-alloc.h> header that has all the variations in one
> place, and that is only included by the two (!) users of these things
> would seem to be a good idea regardless.
> 
> Because <linux/mm.h> is included by pretty much everything. Why do we
> have those alloc_track functions defined in such a common header when
> they are _so_ special?
> 
> Please? I'd obviously like this to be fixed on ppc asap, but I'd also
> like the fix to improve on the current somewhat confusing situation..
> 
> For extra point, the p??_alloc_track() functions could even be
> generated from a macro pattern, because the pattern is pretty much set
> in stone.
> 
> I think the only thing that really differs is the types and the
> PGTBL_xyz_MODIFIED mask, and which entry is tested for "none" (which
> is also the only thing that makes the 5level fixup case different -
> no?

As discussed over in
https://lore.kernel.org/linux-mm/20200604164814.GA7600@kernel.org/,
Mike's "mm: remove __ARCH_HAS_5LEVEL_HACK" patchset
(http://lkml.kernel.org/r/20200414153455.21744-1-rppt@kernel.org) is
expected to fix this.  5level-fixup.h gets removed.

I hope to have that patchset sent over later today.