linux-kernel - Re: [RFC 2/3] mm/vmalloc: add support for __GFP

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Message-id: <163520582122.16092.9250045450947778926@noble.neil.brown.name>
Date:   Tue, 26 Oct 2021 10:50:21 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Uladzislau Rezki" <urezki@...il.com>
Cc:     "Michal Hocko" <mhocko@...e.com>,
        "Uladzislau Rezki" <urezki@...il.com>,
        "Michal Hocko" <mhocko@...e.com>,
        "Linux Memory Management List" <linux-mm@...ck.org>,
        "Dave Chinner" <david@...morbit.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Christoph Hellwig" <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        "LKML" <linux-kernel@...r.kernel.org>,
        "Ilya Dryomov" <idryomov@...il.com>,
        "Jeff Layton" <jlayton@...nel.org>
Subject: Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL

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.

Thanks,
NeilBrown