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Date: Tue, 19 Jun 2018 02:02:55 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()
Message-ID: <20180619090255.GA25522@bombadil.infradead.org>
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To: Jan Kara <jack@suse.cz>
Cc: John Hubbard <jhubbard@nvidia.com>, Dan Williams <dan.j.williams@intel.com>, Christoph Hellwig <hch@lst.de>, Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <john.hubbard@gmail.com>, Michal Hocko <mhocko@kernel.org>, Christopher Lameter <cl@linux.com>, Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, linux-rdma <linux-rdma@vger.kernel.org>

On Tue, Jun 19, 2018 at 10:29:49AM +0200, Jan Kara wrote:
> And for record, the problem with page cache pages is not only that
> try_to_unmap() may unmap them. It is also that page_mkclean() can
> write-protect them. And once PTEs are write-protected filesystems may end
> up doing bad things if DMA then modifies the page contents (DIF/DIX
> failures, data corruption, oopses). As such I don't think that solutions
> based on page reference count have a big chance of dealing with the
> problem.
> 
> And your page flag approach would also need to take page_mkclean() into
> account. And there the issue is that until the flag is cleared (i.e., we
> are sure there are no writers using references from GUP) you cannot
> writeback the page safely which does not work well with your idea of
> clearing the flag only once the page is evicted from page cache (hint, page
> cache page cannot get evicted until it is written back).
> 
> So as sad as it is, I don't see an easy solution here.

Pages which are "got" don't need to be on the LRU list.  They'll be
marked dirty when they're put, so we can use page->lru for fun things
like a "got" refcount.  If we use bit 1 of page->lru for PageGot, we've
got 30/62 bits in the first word and a full 64 bits in the second word.