Re: [PATCH v9 02/14] mm: move page zone helpers from mm.h to mmzone.h

From: Peter Xu <peterx@redhat.com>
To: Alex Sierra <alex.sierra@amd.com>
Cc: rcampbell@nvidia.com, willy@infradead.org, david@redhat.com,
	Felix.Kuehling@amd.com, apopple@nvidia.com,
	amd-gfx@lists.freedesktop.org, linux-xfs@vger.kernel.org,
	linux-mm@kvack.org, jglisse@redhat.com,
	dri-devel@lists.freedesktop.org, jgg@nvidia.com,
	akpm@linux-foundation.org, linux-ext4@vger.kernel.org,
	hch@lst.de
Subject: Re: [PATCH v9 02/14] mm: move page zone helpers from mm.h to mmzone.h
Date: Thu, 15 Jun 2023 15:33:12 -0400	[thread overview]
Message-ID: <ZItneGX+sqg7WApF@x1n> (raw)
In-Reply-To: <20220715150521.18165-3-alex.sierra@amd.com>

Hello, all,

On Fri, Jul 15, 2022 at 10:05:09AM -0500, Alex Sierra wrote:
> +static inline enum zone_type page_zonenum(const struct page *page)
> +{
> +	ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);
> +	return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
> +}

Sorry to hijack this patch - not directly relevant to the movement, but
relevant to this helper, so maybe I can leverage the cc list..

My question is whether page_zonenum() is ready for taking all kinds of tail
pages?

Zone device tail pages all look fine, per memmap_init_zone_device().  The
question was other kinds of usual compound pages, like either thp or
hugetlb.  IIUC page->flags can be uninitialized for those tail pages.

Asking because I noticed it seems possible that page_zonenum() can just
take any random tail page as input, e.g.:

try_grab_folio -> is_pci_p2pdma_page -> is_zone_device_page -> page_zonenum

I'm worried it'll just read fake things, but maybe I just missed something?

Thanks,

-- 
Peter Xu