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

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Date:   Thu, 29 Jun 2017 14:49:01 -0700
From:   "prakash.sangappa" <prakash.sangappa@...cle.com>
To:     Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Michal Hocko <mhocko@...nel.org>, 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 06/29/2017 03:46 AM, Mike Rapoport wrote:
> On Wed, Jun 28, 2017 at 11:23:32AM -0700, Prakash Sangappa wrote:
[...]
>>
>> 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.
>   
> Well, what I had in mind was using a single-process uffd monitor that will
> track all the userfault file descriptors. With UFFD_EVENT_REMOVE this
> process will know what areas are invalid and it will be able to process the
> invalid access in any way it likes, e.g. send SIGBUS to the database
> application.


Use of a monitor process is also an overhead for the database.


>
> If you mmap() and userfaultfd_register() only at the initialization time,
> it might be also possible to avoid sending userfault file descriptors to
> the monitor process with UFFD_FEATURE_EVENT_FORK.

The new processes are always exec'd in the database case and these
processes could be mapping different files. So, not sure if
UFFD_FEATURE_EVENT_FORK will be useful.  Also, it may not be one
process spawning the other new processes.


>
> --
> Sincerely yours,
> Mike.
>