On Tue, Sep 29, 2020 at 10:19:38AM +0200, Michal Hocko wrote:
On Wed 16-09-20 23:43:02, Daniel Vetter wrote:
I can
then figure out whether it's better to risk not spotting issues with
call_rcu vs slapping a memalloc_noio_save/restore around all these
critical section which force-degrades any allocation to GFP_ATOMIC at
did you mean memalloc_noreclaim_* here?
Yeah I picked the wrong one of that family of functions.
most, but has the risk that we run into code that assumes "GFP_KERNEL
never fails for small stuff" and has a decidedly less tested fallback
path than rcu code.
Even if the above then please note that memalloc_noreclaim_* or
PF_MEMALLOC should be used with an extreme care. Essentially only for
internal memory reclaimers. It grants access to _all_ the available
memory so any abuse can be detrimental to the overall system operation.
Allocation failure in this mode means that we are out of memory and any
code relying on such an allocation has to carefuly consider failure.
This is not a random allocation mode.
Agreed, that's why I don't like having these kind of automagic critical
sections. It's a bit a shotgun approach. Paul said that the code would
handle failures, but the problem is that it applies everywhere.
Anyway my understanding is that call_rcu will be reworked and gain a pile
of tricks so that these problems for the callchains leading to call_rcu
all disappear.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
