Re: [PATCH v2 00/33] Separate struct slab from struct page

From: Matthew Wilcox <willy@infradead.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Michal Hocko <mhocko@kernel.org>,
	linux-mm@kvack.org, Andrey Ryabinin <ryabinin.a.a@gmail.com>,
	Alexander Potapenko <glider@google.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>,
	Christoph Lameter <cl@linux.com>, Will Deacon <will@kernel.org>,
	Julia Lawall <julia.lawall@inria.fr>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	x86@kernel.org, Luis Chamberlain <mcgrof@kernel.org>,
	kasan-dev@googlegroups.com, Ingo Molnar <mingo@redhat.com>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	David Rientjes <rientjes@google.com>,
	Nitin Gupta <ngupta@vflare.org>, Marco Elver <elver@google.com>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	cgroups@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	patches@lists.linux.dev, Pekka Enberg <penberg@kernel.org>,
	Minchan Kim <minchan@kernel.org>,
	iommu@lists.linux-foundation.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Woodhouse <dwmw2@infradead.org>,
	Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH v2 00/33] Separate struct slab from struct page
Date: Mon, 20 Dec 2021 01:42:13 +0000	[thread overview]
Message-ID: <Yb/fdYbLunsVYRqQ@casper.infradead.org> (raw)
In-Reply-To: <86617be0-8aa8-67d2-08bd-1e06c3d12785@suse.cz>

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.

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