linux-kernel - Re: [RFC PATCH] userfaultfd: Add feature to request for a signal delivery

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Message-ID: <20170629080910.GC31603@dhcp22.suse.cz>
Date:   Thu, 29 Jun 2017 10:09:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Prakash Sangappa <prakash.sangappa@...cle.com>
Cc:     Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-api@...r.kernel.org
Subject: Re: [RFC PATCH] userfaultfd: Add feature to request for a signal
 delivery

On Wed 28-06-17 11:23:32, Prakash Sangappa wrote:
> 
> 
> On 6/28/17 6:18 AM, Mike Rapoport wrote:
[...]
> >I've just been thinking that maybe it would be possible to use
> >UFFD_EVENT_REMOVE for this case. We anyway need to implement the generation
> >of UFFD_EVENT_REMOVE for the case of hole punching in hugetlbfs for
> >non-cooperative userfaultfd. It could be that it will solve your issue as
> >well.
> >
> 
> Will this result in a signal delivery?
> 
> In the use case described, the database application does not need any event
> for  hole punching. Basically, just a signal for any invalid access to
> mapped area over holes in the file.

OK, but it would be better to think that through for other potential
usecases so that this doesn't end up as a single hugetlb feature. E.g.
what should happen if a regular anonymous memory gets swapped out?
Should we deliver signal as well? How does userspace tell whether this
was a no backing page from unavailable backing page?
-- 
Michal Hocko
SUSE Labs