linux-kernel - Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

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Message-ID: <20190213150632.GB26828@quack2.suse.cz>
Date:   Wed, 13 Feb 2019 16:06:32 +0100
From:   Jan Kara <jack@...e.cz>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Jan Kara <jack@...e.cz>, Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Doug Ledford <dledford@...hat.com>,
        lsf-pc@...ts.linux-foundation.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On Tue 12-02-19 16:55:21, Christopher Lameter wrote:
> On Tue, 12 Feb 2019, Jan Kara wrote:
> 
> > > Isn't that already racy? If the mmap user is fast enough can't it
> > > prevent the page from becoming freed in the first place today?
> >
> > No, it cannot. We block page faulting for the file (via a lock), tear down
> > page tables, free pages and blocks. Then we resume faults and return
> > SIGBUS (if the page ends up being after the new end of file in case of
> > truncate) or do new page fault and fresh block allocation (which can end
> > with SIGBUS if the filesystem cannot allocate new block to back the page).
> 
> Well that is already pretty inconsistent behavior. Under what conditions
> is the SIGBUS occurring without the new fault attempt?

I probably didn't express myself clearly enough. I didn't say that SIGBUS
can occur without a page fault. The evaluation of whether a page would be
beyond EOF, page allocation, and block allocation happen only in response
to a page fault...

> If a new fault is attempted then we have resource constraints that could
> have caused a SIGBUS independently of the truncate. So that case is not
> really something special to be considered for truncation.

Agreed. I was just reacting to Jason's question whether an application
cannot prevent page freeing by being aggressive enough.

								Honza

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