linux-kernel - Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user

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Date:   Tue, 19 Jun 2018 02:02:55 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <john.hubbard@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

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.