linux-ext4 - Re: [PATCH v9 02/14] mm: move page zone helpers from mm.h to mmzone.h

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Date:   Thu, 15 Jun 2023 15:33:12 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alex Sierra <alex.sierra@....com>
Cc:     jgg@...dia.com, david@...hat.com, Felix.Kuehling@....com,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, apopple@...dia.com,
        willy@...radead.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v9 02/14] mm: move page zone helpers from mm.h to mmzone.h

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