From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH v2 00/33] Separate struct slab from struct page
Date: Mon, 20 Dec 2021 01:42:13 +0000
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To: Vlastimil Babka <vbabka-AlSwsSmVLrQ@public.gmane.org>
Cc: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>, Christoph Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Joonsoo Kim <iamjoonsoo.kim-Hm3cg6mZ9cc@public.gmane.org>, Pekka Enberg <penberg-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, patches-cunTk1MwBs/YUNznpcFYbw@public.gmane.org, Alexander Potapenko <glider-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andrey Konovalov <andreyknvl-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Andrey Ryabinin <ryabinin.a.a-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Andy Lutomirski <luto-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Borislav Petkov <bp-Gina5bIWoIWzQB+pC5nmwQ@public.gmane.org>, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Dave Hansen <dave.hansen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>, David Woodhouse <dwmw2-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, Dmitry Vyukov <dvyukov-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, J

On Mon, Dec 20, 2021 at 01:47:54AM +0100, Vlastimil Babka wrote:
> > * mm/slub: Convert print_page_info() to print_slab_info()
> > Do we really need to explicitly convert slab_folio()'s result to (struct folio *)?
> 
> Unfortunately yes, as long as folio_flags() don't take const struct folio *,
> which will need some yak shaving.

In case anyone's interested ...

folio_flags calls VM_BUG_ON_PGFLAGS() which would need its second
argument to be const.

That means dump_page() needs to take a const struct page, which
means __dump_page() needs its argument to be const.

That calls ...

is_migrate_cma_page()
page_mapping()
page_mapcount()
page_ref_count()
page_to_pgoff()
page_to_pfn()
hpage_pincount_available()
head_compound_mapcount()
head_compound_pincount()
compound_order()
PageKsm()
PageAnon()
PageCompound()

... and at that point, I ran out of motivation to track down some parts
of this tarbaby that could be fixed.  I did do:

    mm: constify page_count and page_ref_count
    mm: constify get_pfnblock_flags_mask and get_pfnblock_migratetype
    mm: make compound_head const-preserving
    mm/page_owner: constify dump_page_owner

so some of those are already done.  But a lot of them just need to be
done at the same time.  For example, page_mapping() calls
folio_mapping() which calls folio_test_slab() which calls folio_flags(),
so dump_page() and page_mapping() need to be done at the same time.

One bit that could be broken off easily (I think ...) is PageTransTail()
PageTail(), PageCompound(), PageHuge(), page_to_pgoff() and
page_to_index().  One wrinkle is needed a temporary
TESTPAGEFLAGS_FALSE_CONST.  But I haven't tried it yet.