Re: [ATTEND] many topics

[Michal Hocko <mhocko@kernel.org>]

On Thu 19-01-17 03:05:13, willy@infradead.org wrote:
> On Wed, Jan 18, 2017 at 02:32:43PM +0100, Michal Hocko wrote:
> > On Tue 17-01-17 21:49:45, Matthew Wilcox wrote:
> > [...]
> > > 8. Nailing down exactly what GFP_TEMPORARY means
> > 
> > It's a hint that the page allocator should group those pages together
> > for better fragmentation avoidance. Have a look at e12ba74d8ff3 ("Group
> > short-lived and reclaimable kernel allocations"). Basically it is
> > something like __GFP_MOVABLE for kernel allocations which cannot go to
> > the movable zones.
> 
> Let me rephrase the topic ... Under what conditions should somebody use
> the GFP_TEMPORARY gfp_t?

Most users of slab (kmalloc) do not really have to care. Slab will add
__GFP_RECLAIMABLE to all reclaimable caches automagically AFAIR. The
remaining would have to implement some kind of shrinker to allow the
reclaim.

> Example usages that I have questions about:
> 
> 1. Is it permissible to call kmalloc(GFP_TEMPORARY), or is it only
> for alloc_pages?

kmalloc will use it internally as mentioned above.  I am not even sure
whether direct using of kmalloc(GFP_TEMPORARY) is ok.  I would have to
check the code but I guess it would be just wrong unless you know your
cache is reclaimable.

> I ask because if the slab allocator is unaware of
> GFP_TEMPORARY, then a non-GFP_TEMPORARY allocation may be placed in a
> page allocated with GFP_TEMPORARY and we've just made it meaningless.
> 
> 2. Is it permissible to sleep while holding a GFP_TEMPORARY allocation?
> eg, take a mutex, or wait_for_completion()?

Yes, GFP_TEMPORARY has ___GFP_DIRECT_RECLAIM set so this is by
definition sleepable allocation request.

> 3. Can I make one GFP_TEMPORARY allocation, and then another one?

Not sure I understand. WHy would be a problem?

> 4. Should I disable preemption while holding a GFP_TEMPORARY allocation,
> or are we OK with a task being preempted?

no, it can sleep.

> 5. What about something even longer duration like allocating a kiocb?
> That might take an arbitrary length of time to be freed, but eventually
> the command will be timed out (eg 30 seconds for something that ends up
> going through SCSI).

I do not understand. The reclaimability of the object is in hands of the
respective shrinker...
 
> 6. Or shorter duration like doing a GFP_TEMPORARY allocation, then taking
> a spinlock, which *probably* isn't contended, but you never know.
> 
> 7. I can see it includes __GFP_WAIT so it's not suitable for using from
> interrupt context, but interrupt context might be the place which can
> benefit from it the most.  Or does GFP_ATOMIC's __GFP_HIGH also allow for
> allocation from the movable zone?  Should we have a GFP_TEMPORARY_ATOMIC?

This is where __GFP_RECLAIMABLE should be used as this is the core of
the functionality.
-- 
Michal Hocko
SUSE Labs

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