Hello,
On 11.10.22 12:23, David Hildenbrand wrote:
Sounds to me that all you want is some notifier to be called from
adjust_managed_page_count(). What am I missing?
Notifier will act as an accumulator to report size of change and it
will make things easier for the drivers and users wrt locking.
Notifier is similar to the memory hotplug notifier.
Overall, I am not convinced that there is any value of separating the
value
and the notifier. You can batch both or not batch both. In addition,
as I
mentioned, having two values seems racy.
I have identified two users so far above - may be more to come.
One type needs the value to adjust. Also having the value is necessary
to report it to users and oom. There are options with callbacks and so
on but it will complicate things with no real gain. You are right about
the atomicity but i guess if that's a problem for some user it could
find a way to ensure it. i am yet to find such place.
I haven't followed the whole discussion, but I just wanted to raise that
having a generic mechanism to notify on such changes could be valuable.
For example, virtio-mem also uses adjust_managed_page_count() and might
sometimes not trigger memory hotplug notifiers when adding more memory
(essentially, when it fake-adds memory part of an already added Linux
memory block).
What might make sense is schedule some kind of deferred notification on
adjust_managed_page_count() changes. This way, we could notify without
caring about locking and would naturally batch notifications.
adjust_managed_page_count() users would not require changes.
Making it deferred will bring issues for both the users of the
adjust_managed_page_count and the receivers of the notification -
locking as first. And it is hard to know when the adjustment will
finish, some of the drivers wait and retry in blocks. It will bring
complexity and it will not be possible to convert users in small steps.
Other problem is that there are drivers that do not use
adjust_managed_page_count().
Extending the current memory hotplug notifier is not an option too much
things can break.
I think some new option CONFIG_DYN_MEM_CONSTRAINTS or a better name.
Add a dynmem_notifier that bring size changes to the subscribers.
CONFIG_DYN_MEM_CONSTRAINTS to be able to convert current __init
functions without growing kernel in size when not using it.
One idea about the absolut values in sysfs/proc is to check if the
option is set by the user or it is the same as was calculated at init
time - if it is not set by the user it can be adjusted up/down by
percent of the change in memory - it may require min/max value boundaries.
Does anyone know - if it is possible to know if a sysfs/proc file has
been written to since boot?
--
Regards,
Alexander Atanasov
