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Date:   Tue, 30 Aug 2022 15:05:49 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Alexander Potapenko <glider@google.com>,
        Yu Zhao <yuzhao@google.com>
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        Alexander Viro <viro@zeniv.linux.org.uk>,
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Subject: Re: [PATCH v5 04/44] x86: asm: instrument usercopy in get_user()
 and put_user()
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On Tue, 30 Aug 2022 16:23:44 +0200 Alexander Potapenko <glider@google.com> wrote:

> >                  from init/do_mounts.c:2:
> > ./include/linux/page-flags.h: In function ‘page_fixed_fake_head’:
> > ./include/linux/page-flags.h:226:36: error: invalid use of undefined type ‘const struct page’
> >   226 |             test_bit(PG_head, &page->flags)) {
> >       |                                    ^~
> > ./include/linux/bitops.h:50:44: note: in definition of macro ‘bitop’
> >    50 |           __builtin_constant_p((uintptr_t)(addr) != (uintptr_t)NULL) && \
> >       |                                            ^~~~
> > ./include/linux/page-flags.h:226:13: note: in expansion of macro ‘test_bit’
> >   226 |             test_bit(PG_head, &page->flags)) {
> >       |             ^~~~~~~~
> > ...
> 
> Gotcha, this is a circular dependency: mm_types.h -> sched.h ->
> kmsan.h -> gfp.h -> mmzone.h -> page-flags.h -> mm_types.h, where the
> inclusion of sched.h into mm_types.h was only introduced in "mm:
> multi-gen LRU: support page table walks" - that's why the problem was
> missing in other trees.

Ah, thanks for digging that out.

Yu, that inclusion is regrettable.  I don't think mm_types.h is an
appropriate site for implementing lru_gen_use_mm() anyway.  Adding a
new header is always the right fix for these things.  I'd suggest
adding a new mglru.h (or whatever) and putting most/all of the mglru
material in there.

Also, the addition to kernel/sched/core.c wasn't clearly changelogged,
is uncommented and I doubt if the sched developers know about it, let
alone reviewed it.  Please give them a heads-up.

The addition looks fairly benign, but core context_switch() is the
sort of thing which people get rather defensive about and putting
mm-specific stuff in there might be challenged.  Some quantitative
justification of this optimization would be appropriate.

> In fact sched.h only needs the definitions of `struct
> kmsan_context_state` and `struct kmsan_ctx` from kmsan.h, so I am
> splitting them off into kmsan_types.h to break this circle.
> Doing so also helped catch a couple of missing/incorrect inclusions of
> KMSAN headers in subsystems.