linux-kernel - Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take the mmap

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Date:   Thu, 28 Mar 2019 19:24:04 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take
 the mmap_sem v2

On Thu, Mar 28, 2019 at 04:20:37PM -0700, John Hubbard wrote:
> On 3/28/19 4:05 PM, Jerome Glisse wrote:
> > On Thu, Mar 28, 2019 at 03:43:33PM -0700, John Hubbard wrote:
> >> On 3/28/19 3:40 PM, Jerome Glisse wrote:
> >>> On Thu, Mar 28, 2019 at 03:25:39PM -0700, John Hubbard wrote:
> >>>> On 3/28/19 3:08 PM, Jerome Glisse wrote:
> >>>>> On Thu, Mar 28, 2019 at 02:41:02PM -0700, John Hubbard wrote:
> >>>>>> On 3/28/19 2:30 PM, Jerome Glisse wrote:
> >>>>>>> On Thu, Mar 28, 2019 at 01:54:01PM -0700, John Hubbard wrote:
> >>>>>>>> On 3/25/19 7:40 AM, jglisse@...hat.com wrote:
> >>>>>>>>> From: Jérôme Glisse <jglisse@...hat.com>
> >>>> [...]
> >>>> OK, so let's either drop this patch, or if merge windows won't allow that,
> >>>> then *eventually* drop this patch. And instead, put in a hmm_sanity_check()
> >>>> that does the same checks.
> >>>
> >>> RDMA depends on this, so does the nouveau patchset that convert to new API.
> >>> So i do not see reason to drop this. They are user for this they are posted
> >>> and i hope i explained properly the benefit.
> >>>
> >>> It is a common pattern. Yes it only save couple lines of code but down the
> >>> road i will also help for people working on the mmap_sem patchset.
> >>>
> >>
> >> It *adds* a couple of lines that are misleading, because they look like they
> >> make things safer, but they don't actually do so.
> > 
> > It is not about safety, sorry if it confused you but there is nothing about
> > safety here, i can add a big fat comment that explains that there is no safety
> > here. The intention is to allow the page fault handler that potential have
> > hundred of page fault queue up to abort as soon as it sees that it is pointless
> > to keep faulting on a dying process.
> > 
> > Again if we race it is _fine_ nothing bad will happen, we are just doing use-
> > less work that gonna be thrown on the floor and we are just slowing down the
> > process tear down.
> > 
> 
> In addition to a comment, how about naming this thing to indicate the above 
> intention?  I have a really hard time with this odd down_read() wrapper, which
> allows code to proceed without really getting a lock. It's just too wrong-looking.
> If it were instead named:
> 
> 	hmm_is_exiting()

What about: hmm_lock_mmap_if_alive() ?


> 
> and had a comment about why racy is OK, then I'd be a lot happier. :)

Will add fat comment.

Cheers,
Jérôme