Re: [PATCH 04/16] mm: allow PF_MEMALLOC from softirq context

[Mel Gorman <mgorman@suse.de>]

On Sun, Jul 08, 2012 at 08:12:11PM +0200, Sebastian Andrzej Siewior wrote:
> On Wed, Jun 27, 2012 at 09:26:14AM +0100, Mel Gorman wrote:
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > index b6c0727..5c6d9c6 100644
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -2265,7 +2265,11 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> > > >  	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> > > >  		if (gfp_mask & __GFP_MEMALLOC)
> > > >  			alloc_flags |= ALLOC_NO_WATERMARKS;
> > > > -		else if (likely(!(gfp_mask & __GFP_NOMEMALLOC)) && !in_interrupt())
> > > > +		else if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
> > > > +			alloc_flags |= ALLOC_NO_WATERMARKS;
> > > > +		else if (!in_interrupt() &&
> > > > +				((current->flags & PF_MEMALLOC) ||
> > > > +				 unlikely(test_thread_flag(TIF_MEMDIE))))
> > > >  			alloc_flags |= ALLOC_NO_WATERMARKS;
> > > >  	}
> > > 
> > > You allocate in RX path with __GFP_MEMALLOC and your sk->sk_allocation has
> > > also __GFP_MEMALLOC set. That means you should get ALLOC_NO_WATERMARKS in
> > > alloc_flags.
> > 
> > In the cases where they are annotated correctly, yes. It is recordeed if
> > the page gets allocated from the PFMEMALLOC reserves. If the received
> > packet is not SOCK_MEMALLOC and the page was allocated from PFMEMALLOC
> > reserves it is then discarded and the packet must be retransmitted.
> 
> Let me try again:
> - lets assume your allocation happens with alloc_page(), without
>   __GFP_MEMALLOC in GFP_FLAGS and with PF_MEMALLOC in current->flags. Now
>   you may get memory which you wouldn't receive otherwise (without
>   PF_MEMALLOC). Okay, understood. So you don't have to annotate each page
>   allocation in your receive path for instance as long as the process has the
>   flag set.

Yes.

> - lets assume your allocation happens with kmalloc() without __GFP_MEMALLOC
>   and current->flags has PF_MEMALLOC ORed and your SLAB pool is empty. This
>   forces SLAB to allocate more pages from the buddy allocator with it will
>   receive more likely (due to ->current->flags + PF_MEMALLOC) but SLAB will
>   drop this extra memory because the page has ->pf_memory (or something like
>   that) set and the GFP_FLAGS do not have __GFP_MEMALLOC set.
> 

It's recorded if the slab page was allocated from PFMEMALLOC reserves (see
patch 2 from the swap over NBD series). slab will use this page for objects
but only allocate them to callers that pass a gfp_pfmemalloc_allowed() check.
kmalloc() users with either __GFP_MEMALLOC or PF_MEMALLOC will get
the pages they need but they will not "leak" to !_GFP_MEMALLOC users as
that would potentially deadlock.

-- 
Mel Gorman
SUSE Labs

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