Re: [PATCH 0/3] userfaultfd: non-cooperative: syncronous events

[Andrew Morton]

On Tue, 27 Feb 2018 10:19:49 +0200 Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> wrote:

> Hi,
> 
> These patches add ability to generate userfaultfd events so that their
> processing will be synchronized with the non-cooperative thread that caused
> the event.
> 
> In the non-cooperative case userfaultfd resumes execution of the thread
> that caused an event when the notification is read() by the uffd monitor.
> In some cases, like, for example, madvise(MADV_REMOVE), it might be
> desirable to keep the thread that caused the event suspended until the
> uffd monitor had the event handled to avoid races between the thread that
> caused the and userfaultfd ioctls.
> 
> Theses patches extend the userfaultfd API with an implementation of
> UFFD_EVENT_REMOVE_SYNC that allows to keep the thread that triggered
> UFFD_EVENT_REMOVE until the uffd monitor would not wake it explicitly.

"might be desirable" is a bit weak.  It might not be desirable, too ;)

_Is_ it desirable?  What are the use-cases and what is the end-user
benefit?