Re: [PATCH] thp, mm: remove comments on serializion of THP split vs. gup_fast

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Fri, 26 Feb 2016 12:06:50 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Feb 25, 2016 at 10:50:14PM -0800, Hugh Dickins wrote:
> 
> > For example, see the fallback tlb_remove_table_one() in mm/memory.c:
> > that one uses smp_call_function() sending IPI to all CPUs concerned,
> > without waiting an RCU grace period at all.
> 
> The better comment is with mmu_table_batch.
> 
> Its been too long for me to fully remember, nor have I really paid much
> attention to this code in the past few years, so any memory I might have
> had might be totally wrong.
> 
> But relying on rcu_read_lock_sched() and friends would mean replacing
> that smp_call_function() with synchronize_sched().

That makes sense, just tried that together with a big fat printk to see if
we hit that out-of-memory condition in the page table freeing code.
The system is swapping like mad but no message so far.
 
> A real quick look at the current code seems to suggest that _might_ just
> work, but note that that will be slower, RT and HPC people will like you
> for it though.
> 
> So it depends on how hard we hit that special, totally out of memory,
> case, and if we care about some performance if we do.

If the system is out of memory bad enough for the page allocation to fail
an additional synchronize_sched() call probably won't hurt too much. Most
of the time we'll be waiting for I/O anyway.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>