Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL

From: Michal Hocko <mhocko@suse.com>
To: NeilBrown <neilb@suse.de>
Cc: Uladzislau Rezki <urezki@gmail.com>,
	Linux Memory Management List <linux-mm@kvack.org>,
	Dave Chinner <david@fromorbit.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel@vger.kernel.org,
	LKML <linux-kernel@vger.kernel.org>,
	Ilya Dryomov <idryomov@gmail.com>,
	Jeff Layton <jlayton@kernel.org>
Subject: Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL
Date: Tue, 26 Oct 2021 09:16:57 +0200	[thread overview]
Message-ID: <YXeraV5idipgWDB+@dhcp22.suse.cz> (raw)
In-Reply-To: <163520582122.16092.9250045450947778926@noble.neil.brown.name>

On Tue 26-10-21 10:50:21, Neil Brown wrote:
> On Mon, 25 Oct 2021, Uladzislau Rezki wrote:
> > On Fri, Oct 22, 2021 at 09:49:08AM +1100, NeilBrown wrote:
> > > However I'm not 100% certain, and the behaviour might change in the
> > > future.  So having one place (the definition of memalloc_retry_wait())
> > > where we can change the sleeping behaviour if the alloc_page behavour
> > > changes, would be ideal.  Maybe memalloc_retry_wait() could take a
> > > gfpflags arg.
> > > 
> > At sleeping is required for __get_vm_area_node() because in case of lack
> > of vmap space it will end up in tight loop without sleeping what is
> > really bad.
> > 
> So vmalloc() has two failure modes.  alloc_page() failure and
> __alloc_vmap_area() failure.  The caller cannot tell which...
> 
> Actually, they can.  If we pass __GFP_NOFAIL to vmalloc(), and it fails,
> then it must have been __alloc_vmap_area() which failed.
> What do we do in that case?
> Can we add a waitq which gets a wakeup when __purge_vmap_area_lazy()
> finishes?
> If we use the spinlock from that waitq in place of free_vmap_area_lock,
> then the wakeup would be nearly free if no-one was waiting, and worth
> while if someone was waiting.

Is this really required to be part of the initial support?
-- 
Michal Hocko
SUSE Labs