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Date:   Tue, 3 Jul 2018 17:48:13 +0000
From:   Christopher Lameter <cl@linux.com>
To:     John Hubbard <jhubbard@nvidia.com>
cc:     Jan Kara <jack@suse.cz>, john.hubbard@gmail.com,
        Matthew Wilcox <willy@infradead.org>,
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        LKML <linux-kernel@vger.kernel.org>,
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        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 5/6] mm: track gup pages with page->dma_pinned_*
 fields
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On Tue, 3 Jul 2018, John Hubbard wrote:

> The page->_refcount field is used normally, in addition to the dma_pinned_count.
> But the problem is that, unless the caller knows what kind of page it is,
> the page->dma_pinned_count cannot be looked at, because it is unioned with
> page->lru.prev.  page->dma_pinned_flags, at least starting at bit 1, are
> safe to look at due to pointer alignment, but now you cannot atomically
> count...
>
> So this seems unsolvable without having the caller specify that it knows the
> page type, and that it is therefore safe to decrement page->dma_pinned_count.
> I was hoping I'd found a way, but clearly I haven't. :)

Try to find some way to indicate that the page is pinned by using some of
the existing page flags? There is already an MLOCK flag. Maybe some
creativity with that can lead to something (but then the MLOCKed pages are
on the unevictable LRU....). cgroups used to have something called struct
page_ext. Oh its there in linux/mm/page_ext.c.