Date: Wed, 25 Apr 2018 08:43:32 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
cc: Richard Weinberger <richard@nod.at>,
        LKML <linux-kernel@vger.kernel.org>,
        Artem Bityutskiy <dedekind1@gmail.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Brian Norris <computersforpeace@gmail.com>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        Marek Vasut <marek.vasut@gmail.com>,
        Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Bob Peterson <rpeterso@redhat.com>,
        Trond Myklebust <trond.myklebust@primarydata.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        linux-mtd@lists.infradead.org, linux-ext4@vger.kernel.org,
        cluster-devel@redhat.com, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: vmalloc with GFP_NOFS
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On Tue, 24 Apr 2018, Michal Hocko wrote:

> On Tue 24-04-18 19:17:12, Mikulas Patocka wrote:
> > 
> > 
> > On Tue, 24 Apr 2018, Michal Hocko wrote:
> > 
> > > On Wed 25-04-18 00:18:40, Richard Weinberger wrote:
> > > > Am Dienstag, 24. April 2018, 21:28:03 CEST schrieb Michal Hocko:
> > > > > > Also only for debugging.
> > > > > > Getting rid of vmalloc with GFP_NOFS in UBIFS is no big problem.
> > > > > > I can prepare a patch.
> > > > > 
> > > > > Cool!
> > > > > 
> > > > > Anyway, if UBIFS has some reclaim recursion critical sections in general
> > > > > it would be really great to have them documented and that is where the
> > > > > scope api is really handy. Just add the scope and document what is the
> > > > > recursion issue. This will help people reading the code as well. Ideally
> > > > > there shouldn't be any explicit GFP_NOFS in the code.
> > > > 
> > > > So in a perfect world a filesystem calls memalloc_nofs_save/restore and
> > > > always uses GFP_KERNEL for kmalloc/vmalloc?
> > > 
> > > Exactly! And in a dream world those memalloc_nofs_save act as a
> > > documentation of the reclaim recursion documentation ;)
> > > -- 
> > > Michal Hocko
> > > SUSE Labs
> > 
> > BTW. should memalloc_nofs_save and memalloc_noio_save be merged into just 
> > one that prevents both I/O and FS recursion?
> 
> Why should FS usage stop IO altogether?

Because the IO may reach loop and loop may redirect it to the same 
filesystem that is running under memalloc_nofs_save and deadlock.

> > memalloc_nofs_save allows submitting bios to I/O stack and the bios 
> > created under memalloc_nofs_save could be sent to the loop device and the 
> > loop device calls the filesystem...
> 
> Don't those use NOIO context?

What do you mean?

Mikulas