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

[CC John, the thread started
http://lkml.kernel.org/r/9363561f-a9cd-7ab6-9c11-ab9a99dc89f1-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org]

On Thu 29-06-17 14:41:22, prakash.sangappa wrote:

On 06/29/2017 01:09 AM, Michal Hocko wrote:

quoted

On Wed 28-06-17 11:23:32, Prakash Sangappa wrote:

quoted

On 6/28/17 6:18 AM, Mike Rapoport wrote:

[...]

quoted

quoted

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?

This may not be useful in all cases. Potential, it could be used
with use of mlock() on anonymous memory to ensure any access
to memory that is not locked is caught, again for robustness
purpose.

The thing I wanted to point out is that not only this should be a single
usecase thing (I believe others will pop out as well - see below) but it
should also be well defined as this is a user visible API. Please try to
write a patch to the userfaultfd man page to clarify the exact semantic.
This should help the further discussion.

As an aside, I rememeber that prior to MADV_FREE there was long
discussion about lazy freeing of memory from userspace. Some users
wanted to be signalled when their memory was freed by the system so that
they could rebuild the original content (e.g. uncompressed images in
memory). It seems like MADV_FREE + this signalling could be used for
that usecase. John would surely know more about those usecases.
-- 
Michal Hocko
SUSE Labs