From: Jason Gunthorpe <jgg@ziepe.ca>
To: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
John Hubbard <jhubbard@nvidia.com>,
john.hubbard@gmail.com, Matthew Wilcox <willy@infradead.org>,
Michal Hocko <mhocko@kernel.org>,
Christopher Lameter <cl@linux.com>,
Dan Williams <dan.j.williams@intel.com>,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
linux-rdma <linux-rdma@vger.kernel.org>,
linux-fsdevel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
Jerome Glisse <jglisse@redhat.com>,
Christoph Hellwig <hch@infradead.org>,
Ralph Campbell <rcampbell@nvidia.com>
Subject: Re: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder versions
Date: Thu, 11 Oct 2018 07:20:13 -0600 [thread overview]
Message-ID: <20181011132013.GA5968@ziepe.ca> (raw)
In-Reply-To: <20181011084929.GB8418@quack2.suse.cz>
On Thu, Oct 11, 2018 at 10:49:29AM +0200, Jan Kara wrote:
> > This is a real worry. If someone uses a mistaken put_page() then how
> > will that bug manifest at runtime? Under what set of circumstances
> > will the kernel trigger the bug?
>
> At runtime such bug will manifest as a page that can never be evicted from
> memory. We could warn in put_page() if page reference count drops below
> bare minimum for given user pin count which would be able to catch some
> issues but it won't be 100% reliable. So at this point I'm more leaning
> towards making get_user_pages() return a different type than just
> struct page * to make it much harder for refcount to go wrong...
At least for the infiniband code being used as an example here we take
the struct page from get_user_pages, then stick it in a sgl, and at
put_page time we get the page back out of the sgl via sg_page()
So type safety will not help this case... I wonder how many other
users are similar? I think this is a pretty reasonable flow for DMA
with user pages.
Jason
next prev parent reply other threads:[~2018-10-11 13:20 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-08 21:16 [PATCH v4 0/3] get_user_pages*() and RDMA: first steps john.hubbard
2018-10-08 21:16 ` [PATCH v4 1/3] mm: get_user_pages: consolidate error handling john.hubbard
2018-10-09 0:05 ` Andrew Morton
2018-10-08 21:16 ` [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder versions john.hubbard
2018-10-09 0:14 ` Andrew Morton
2018-10-09 8:30 ` Jan Kara
2018-10-09 23:20 ` Andrew Morton
2018-10-10 0:32 ` John Hubbard
2018-10-10 23:43 ` Andrew Morton
2018-10-10 0:42 ` John Hubbard
2018-10-10 8:59 ` Jan Kara
2018-10-10 23:23 ` John Hubbard
2018-10-11 8:42 ` Jan Kara
2018-10-10 23:45 ` Andrew Morton
2018-10-11 8:49 ` Jan Kara
2018-10-11 13:20 ` Jason Gunthorpe [this message]
2018-10-12 1:23 ` John Hubbard
2018-10-12 3:53 ` John Hubbard
2018-10-18 10:19 ` Jan Kara
2018-11-05 7:25 ` John Hubbard
2018-10-22 19:43 ` Jason Gunthorpe
2018-11-05 7:17 ` John Hubbard
2018-11-05 8:37 ` Jan Kara
2018-10-08 21:16 ` [PATCH v4 3/3] infiniband/mm: convert put_page() to put_user_page*() john.hubbard
2018-10-09 9:52 ` kbuild test robot
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