From: Jason Gunthorpe <jgg@ziepe.ca>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Jan Kara <jack@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
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: Mon, 22 Oct 2018 13:43:29 -0600 [thread overview]
Message-ID: <20181022194329.GG30059@ziepe.ca> (raw)
In-Reply-To: <97e89e08-5b94-240a-56e9-ece2b91f6dbc@nvidia.com>
On Thu, Oct 11, 2018 at 06:23:24PM -0700, John Hubbard wrote:
> On 10/11/18 6:20 AM, Jason Gunthorpe wrote:
> > 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.
> >
>
> That is true. The infiniband code, fortunately, never mixes the two page
> types into the same pool (or sg list), so it's actually an easier example
> than some other subsystems. But, yes, type safety doesn't help there. I can
> take a moment to look around at the other areas, to quantify how much a type
> safety change might help.
Are most (all?) of the places working with SGLs?
Maybe we could just have a 'get_user_pages_to_sgl' and 'put_pages_sgl'
sort of interface that handled all this instead of trying to make
something that is struct page based?
It seems easier to get an extra bit for user/!user in the SGL
datastructure?
Jason
next prev parent reply other threads:[~2018-10-22 19:43 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
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 [this message]
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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