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

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Message-ID: <20180627170246.qfvucs72seqabaef@quack2.suse.cz>
Date:   Wed, 27 Jun 2018 19:02:46 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <john.hubbard@...il.com>,
        Matthew Wilcox <willy@...radead.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 Wed 27-06-18 08:57:18, Jason Gunthorpe wrote:
> On Wed, Jun 27, 2018 at 02:42:55PM +0200, Jan Kara wrote:
> > On Wed 27-06-18 13:59:27, Michal Hocko wrote:
> > > On Wed 27-06-18 13:53:49, Jan Kara wrote:
> > > > On Wed 27-06-18 13:32:21, Michal Hocko wrote:
> > > [...]
> > > > > Appart from that, do we really care about 32b here? Big DIO, IB users
> > > > > seem to be 64b only AFAIU.
> > > > 
> > > > IMO it is a bad habit to leave unpriviledged-user-triggerable oops in the
> > > > kernel even for uncommon platforms...
> > > 
> > > Absolutely agreed! I didn't mean to keep the blow up for 32b. I just
> > > wanted to say that we can stay with a simple solution for 32b. I thought
> > > the g-u-p-longterm has plugged the most obvious breakage already. But
> > > maybe I just misunderstood.
> > 
> > Most yes, but if you try hard enough, you can still trigger the oops e.g.
> > with appropriately set up direct IO when racing with writeback / reclaim.
> 
> gup longterm is only different from normal gup if you have DAX and few
> people do, which really means it doesn't help at all.. AFAIK??

Right, what I wrote works only for DAX. For non-DAX situation g-u-p
longterm does not currently help at all. Sorry for confusion.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR