Re: [PATCH] mm/vmalloc: Keep a separate lazy-free list

[Chris Wilson]

On Thu, Apr 14, 2016 at 04:44:48PM +0200, Roman Peniaev wrote:
> On Thu, Apr 14, 2016 at 3:49 PM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Apr 14, 2016 at 03:13:26PM +0200, Roman Peniaev wrote:
> >> Hi, Chris.
> >>
> >> Is it made on purpose not to drop VM_LAZY_FREE flag in
> >> __purge_vmap_area_lazy()?  With your patch va->flags
> >> will have two bits set: VM_LAZY_FREE | VM_LAZY_FREEING.
> >> Seems it is not that bad, because all other code paths
> >> do not care, but still the change is not clear.
> >
> > Oh, that was just a bad deletion.
> >
> >> Also, did you consider to avoid taking static purge_lock
> >> in __purge_vmap_area_lazy() ? Because, with your change
> >> it seems that you can avoid taking this lock at all.
> >> Just be careful when you observe llist as empty, i.e.
> >> nr == 0.
> >
> > I admit I only briefly looked at the lock. I will be honest and say I
> > do not fully understand the requirements of the sync/force_flush
> > parameters.
> 
> if sync:
>    o I can wait for other purge in progress
>       (do not care if purge_lock is dropped)
> 
>    o purge fragmented blocks
> 
> if force_flush:
>    o even nothing to purge, flush TLB, which is costly.
>     (again sync-like is implied)
> 
> > purge_fragmented_blocks() manages per-cpu lists, so that looks safe
> > under its own rcu_read_lock.
> >
> > Yes, it looks feasible to remove the purge_lock if we can relax sync.
> 
> what is still left is waiting on vmap_area_lock for !sync mode.
> but probably is not that bad.

Ok, that's bit beyond my comfort zone with a patch to change the free
list handling. I'll chicken out for the time being, atm I am more
concerned that i915.ko may call set_page_wb() frequently on individual
pages.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre