linux-kernel - Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages

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Date:   Tue, 30 Mar 2021 10:49:53 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Joao Martins <joao.m.martins@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Shiyang Ruan <ruansy.fnst@...itsu.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        david <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the
 get_user_pages_fast() path

On Mon, Mar 29, 2021 at 04:24:19PM -0700, Dan Williams wrote:
> On Thu, Mar 25, 2021 at 7:34 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Thu, Mar 18, 2021 at 10:03:06AM -0700, Dan Williams wrote:
> > > Yes. I still need to answer the question of whether mapping
> > > invalidation triggers longterm pin holders to relinquish their hold,
> > > but that's a problem regardless of whether gup-fast is supported or
> > > not.
> >
> > It does not, GUP users do not interact with addres_space or mmu
> > notifiers
> 
> Ok, but the SIGKILL from the memory_failure() will drop the pin?

In the general case I doubt it..

I think this is fine, we shouldn't expect unplugging a driver to not
block if there are open users.

Demading DMA users to be able to shoot down access to system memory is
a really big kernel change.

Jason