[PATCH 2/5] kernel.h: Add non_block_start/end()

[Michal Hocko]

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