bhyve: reset event after blockif resize

[rew]

Summary

After a resize event is triggered, reset it; this prevents the resize
(NOTE_ATTRIB) event from being triggered more than once.

Currently, the blockif resize functionality is the only user of
mevent_add_flags() - so this change won't affect other mevent users.

I noticed this when bhyve was consuming a 100% of the cpu after a resize
event was triggered.

[jhb]

I don't know that mevent_add_flags is really the right layer to fix this vs providing some way to explicitly mark an event as EV_CLEAR. It could be that all EVFILT_VNODE should be EV_CLEAR? That is, perhaps have mevent_kq_flags set EV_CLEAR in the flags it returns if me_type == EVF_VNODE. (I looked at the kevent(2) manpage and I do think we would want EV_CLEAR for any EVFILT_VNODE events.)

[rew]

In D33326#753476, @jhb wrote:

I don't know that mevent_add_flags is really the right layer to fix this

Agreed - blockif_register_resize_callback() could call mevent_add_state(), but then sys/event.h would have to be included in block_if.c

It could be that all EVFILT_VNODE should be EV_CLEAR?

maybe, most uses of EVFILT_VNODE in base would agree with that.

The man page stated that some filters (e.g., EVFILT_SIGNAL and EVFILT_TIMER) automatically set EV_CLEAR, which led me to believe that event filters that do not set EV_CLEAR must have use-cases where the event filter doesn't want to clear the event.

[rew]

Do you think it would be preferable to set EV_CLEAR in mevent_kq_flags as opposed to using mevent_add_state() directly from blockif_register_resize_callback()?

[jhb]

In D33326#753714, @rew wrote:

Do you think it would be preferable to set EV_CLEAR in mevent_kq_flags as opposed to using mevent_add_state() directly from blockif_register_resize_callback()?

So in theory mevent is a somewhat portable layer that isn't kevent specific. bhyve on illumos doesn't use kevent but is able to just replace mevent.c. Exposing mevent_add_state would beak that I think, so I would prefer changing mevent_kq_fflags and avoiding changing block_if.c in this case (so that the EV_CLEAR is a property of EVF_VNODE that is provided by mevent)

[rew]

set EV_CLEAR flag when EVFILT_VNODE filter is used

[jhb]

usr.sbin/bhyve/mevent.c

178

I would perhaps make this stateless (not modifying mevp) and instead do something like:

int state;

state = mevp->me_state;
if (mevp->me_type == EVF_VNODE)
    state |= EV_CLEAR;
return (state);

[rew]

don't modify mevp, just return the translated flag value

use the name retval and format this function to be consistent with the
function above and below it.

[jhb]

Thanks for fixing this!

[afedorov]

I have this problem in STABLE-13 with disk image located on NFS share.

This patch fixes the problem.

@rew , are you planning on making an MFC to 13-STABLE?

[rew]

In D33326#776604, @afedorov wrote:

I have this problem in STABLE-13 with disk image located on NFS share.

This patch fixes the problem.

@rew , are you planning on making an MFC to 13-STABLE?

done, thanks for the reminder