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From: Michal Hocko <mhocko@kernel.org>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org,
	"DRI Development" <dri-devel@lists.freedesktop.org>,
	"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"David Rientjes" <rientjes@google.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Masahiro Yamada" <yamada.masahiro@socionext.com>,
	"Wei Wang" <wvw@google.com>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Jann Horn" <jannh@google.com>, "Feng Tang" <feng.tang@intel.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Randy Dunlap" <rdunlap@infradead.org>,
	"Daniel Vetter" <daniel.vetter@intel.com>
Subject: Re: [PATCH 2/5] kernel.h: Add non_block_start/end()
Date: Thu, 15 Aug 2019 15:21:27 +0200
Message-ID: <20190815132127.GI9477@dhcp22.suse.cz> (raw)
In-Reply-To: <20190815122344.GA21596@ziepe.ca>

On Thu 15-08-19 09:23:44, Jason Gunthorpe wrote:
> On Thu, Aug 15, 2019 at 08:58:29AM +0200, Daniel Vetter wrote:
> > On Wed, Aug 14, 2019 at 08:58:05PM -0300, Jason Gunthorpe wrote:
> > > On Wed, Aug 14, 2019 at 10:20:24PM +0200, Daniel Vetter wrote:
> > > > In some special cases we must not block, but there's not a
> > > > spinlock, preempt-off, irqs-off or similar critical section already
> > > > that arms the might_sleep() debug checks. Add a non_block_start/end()
> > > > pair to annotate these.
> > > > 
> > > > This will be used in the oom paths of mmu-notifiers, where blocking is
> > > > not allowed to make sure there's forward progress. Quoting Michal:
> > > > 
> > > > "The notifier is called from quite a restricted context - oom_reaper -
> > > > which shouldn't depend on any locks or sleepable conditionals. The code
> > > > should be swift as well but we mostly do care about it to make a forward
> > > > progress. Checking for sleepable context is the best thing we could come
> > > > up with that would describe these demands at least partially."
> > > 
> > > But this describes fs_reclaim_acquire() - is there some reason we are
> > > conflating fs_reclaim with non-sleeping?
> > 
> > No idea why you tie this into fs_reclaim. We can definitly sleep in there,
> > and for e.g. kswapd (which also wraps everything in fs_reclaim) we're
> > event supposed to I thought. To make sure we can get at the last bit of
> > memory by flushing all the queues and waiting for everything to be cleaned
> > out.
> 
> AFAIK the point of fs_reclaim is to prevent "indirect dependency upon
> the page allocator" ie a justification that was given this !blockable
> stuff.
> 
> For instance:
> 
>   fs_reclaim_acquire()
>   kmalloc(GFP_KERNEL) <- lock dep assertion
> 
> And further, Michal's concern about indirectness through locks is also
> handled by lockdep:
> 
>        CPU0                                 CPU1
>                                         mutex_lock()
>                                         kmalloc(GFP_KERNEL)
>                                         mutex_unlock()
>   fs_reclaim_acquire()
>   mutex_lock() <- lock dep assertion
> 
> In other words, to prevent recursion into the page allocator you use
> fs_reclaim_acquire(), and lockdep verfies it in its usual robust way.

fs_reclaim_acquire is about FS/IO recursions IIUC. We are talking about
any !GFP_NOWAIT allocation context here and any {in}direct dependency on
it. Whether fs_reclaim_acquire can be reused for that I do not know
because I am not familiar with the lockdep machinery enough
 
> I asked Tejun about this once in regards to WQ_MEM_RECLAIM and he
> explained that it means you can't call the allocator functions in a
> way that would recurse into reclaim (ie instead use instead GFP_ATOMIC, or
> tolerate allocation failure, or various other things).
> 
> So, the reason I bring it up is half the justifications you posted for
> blockable had to do with not recursing into reclaim and deadlocking,
> and didn't seem to have much to do with blocking.
> 
> I'm asking if *non-blocking* is really the requirement or if this is
> just the usual 'do not deadlock on the allocator' thing reclaim paths
> alread have?

No, non-blocking is a very coarse approximation of what we really need.
But it should give us even a stronger condition. Essentially any sleep
other than a preemption shouldn't be allowed in that context.
-- 
Michal Hocko
SUSE Labs


  reply index

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-14 20:20 [PATCH 0/5] hmm & mmu_notifier debug/lockdep annotations Daniel Vetter
2019-08-14 20:20 ` [PATCH 1/5] mm: Check if mmu notifier callbacks are allowed to fail Daniel Vetter
2019-08-14 22:14   ` Andrew Morton
2019-08-14 23:22     ` Jason Gunthorpe
2019-08-14 23:34     ` Ralph Campbell
2019-08-16 17:19   ` Jason Gunthorpe
2019-08-14 20:20 ` [PATCH 2/5] kernel.h: Add non_block_start/end() Daniel Vetter
2019-08-14 20:45   ` Andrew Morton
2019-08-15  6:52     ` Daniel Vetter
2019-08-15  8:44     ` Michal Hocko
2019-08-15 13:04       ` Jason Gunthorpe
2019-08-15 13:12         ` Daniel Vetter
2019-08-15 14:37           ` Jason Gunthorpe
2019-08-15 14:43             ` Daniel Vetter
2019-08-15 15:10               ` Jason Gunthorpe
2019-08-15 16:25                 ` Daniel Vetter
2019-08-15 17:35                   ` Jason Gunthorpe
2019-08-15 17:39                     ` Jerome Glisse
2019-08-15 18:01                       ` Jason Gunthorpe
2019-08-15 18:27                         ` Jerome Glisse
2019-08-15 18:57                           ` Jason Gunthorpe
2019-08-15 16:32                 ` Jerome Glisse
2019-08-15 17:16                   ` Jason Gunthorpe
2019-08-15 17:21                     ` Daniel Vetter
2019-08-15 17:35                       ` Jerome Glisse
2019-08-15 13:24         ` Michal Hocko
2019-08-15 22:15       ` Andrew Morton
2019-08-16  8:24         ` Michal Hocko
2019-08-14 23:58   ` Jason Gunthorpe
2019-08-15  6:58     ` Daniel Vetter
2019-08-15 12:23       ` Jason Gunthorpe
2019-08-15 13:21         ` Michal Hocko [this message]
2019-08-15 14:12           ` Jason Gunthorpe
2019-08-15 16:00             ` Michal Hocko
2019-08-15 16:56               ` Jason Gunthorpe
2019-08-15 17:11                 ` Jerome Glisse
2019-08-15 17:17                   ` Jason Gunthorpe
2019-08-15 17:42                 ` Michal Hocko
2019-08-15 17:57                   ` Jerome Glisse
2019-08-15 18:24                   ` Jason Gunthorpe
2019-08-15 19:05                     ` Michal Hocko
2019-08-15 19:18                       ` Jason Gunthorpe
2019-08-15 19:35                         ` Michal Hocko
2019-08-15 20:13                           ` Jason Gunthorpe
2019-08-16  8:10                             ` Michal Hocko
2019-08-16 12:19                               ` Jason Gunthorpe
2019-08-16 12:26                                 ` Michal Hocko
2019-08-15 20:16                           ` [Intel-gfx] " Daniel Vetter
2019-08-15 20:27                             ` Jason Gunthorpe
2019-08-15 20:49                               ` Daniel Vetter
2019-08-16  1:00                                 ` Jason Gunthorpe
2019-08-16  6:20                                   ` Daniel Vetter
2019-08-16 12:12                                     ` Jason Gunthorpe
2019-08-16 14:11                                       ` Daniel Vetter
2019-08-16 14:38                                         ` Jason Gunthorpe
2019-08-16 16:36                                           ` Daniel Vetter
2019-08-16 16:54                                             ` Jason Gunthorpe
2019-08-16  8:27                             ` Michal Hocko
2019-08-14 20:20 ` [PATCH 3/5] mm, notifier: Catch sleeping/blocking for !blockable Daniel Vetter
2019-08-15  0:00   ` Jason Gunthorpe
2019-08-15  7:02     ` Daniel Vetter
     [not found]       ` <20190815123556.GB21596@ziepe.ca>
2019-08-17 16:09         ` Daniel Vetter
2019-08-14 20:20 ` [PATCH 4/5] mm, notifier: Add a lockdep map for invalidate_range_start Daniel Vetter
2019-08-15  0:09   ` Jason Gunthorpe
2019-08-15  7:10     ` Daniel Vetter
2019-08-15 12:53       ` Jason Gunthorpe
2019-08-14 20:20 ` [PATCH 5/5] mm/hmm: WARN on illegal ->sync_cpu_device_pagetables errors Daniel Vetter
2019-08-15  0:11   ` Jason Gunthorpe
2019-08-15  7:14     ` Daniel Vetter

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