Date:   Tue, 3 Jan 2017 21:40:14 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, rientjes@...gle.com,
        mgorman@...e.de, hillf.zj@...baba-inc.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3 -v3] GFP_NOFAIL cleanups

On Tue 03-01-17 23:38:30, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 03-01-17 10:36:31, Tetsuo Handa wrote:
> > [...]
> > > I'm OK with "[PATCH 1/3] mm: consolidate GFP_NOFAIL checks in the allocator
> > > slowpath" given that we describe that we make __GFP_NOFAIL stronger than
> > > __GFP_NORETRY with this patch in the changelog.
> > 
> > Again. __GFP_NORETRY | __GFP_NOFAIL is nonsense! I do not really see any
> > reason to describe all the nonsense combinations of gfp flags.
> 
> Before [PATCH 1/3]:
> 
>   __GFP_NORETRY is used as "Do not invoke the OOM killer. Fail allocation
>   request even if __GFP_NOFAIL is specified if direct reclaim/compaction
>   did not help."
> 
>   __GFP_NOFAIL is used as "Never fail allocation request unless __GFP_NORETRY
>   is specified even if direct reclaim/compaction did not help."
> 
> After [PATCH 1/3]:
> 
>   __GFP_NORETRY is used as "Do not invoke the OOM killer. Fail allocation
>   request unless __GFP_NOFAIL is specified."
> 
>   __GFP_NOFAIL is used as "Never fail allocation request even if direct
>   reclaim/compaction did not help. Invoke the OOM killer unless __GFP_NORETRY is
>   specified."
> 
> Thus, __GFP_NORETRY | __GFP_NOFAIL perfectly makes sense as
> "Never fail allocation request if direct reclaim/compaction did not help.
> But do not invoke the OOM killer even if direct reclaim/compaction did not help."

Stop this! Seriously... This is just wasting time...

 * __GFP_NORETRY: The VM implementation must not retry indefinitely and will
 *   return NULL when direct reclaim and memory compaction have failed to allow
 *   the allocation to succeed.  The OOM killer is not called with the current
 *   implementation.

 * __GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
 *   cannot handle allocation failures. New users should be evaluated carefully
 *   (and the flag should be used only when there is no reasonable failure
 *   policy) but it is definitely preferable to use the flag rather than
 *   opencode endless loop around allocator.

Can you see how the two are asking for opposite behavior?  Asking for
not retrying for ever and not failing and rather retrying for ever
simply doesn't make any sense in any reasonable universe I can think
of. Therefore I think that it is fair to say that behavior is undefined
when both are specified.

Considering there are _no_ users which would do that any further
discussion about this is just pointless and I will not respond to any
further emails in this direction.

This is just ridiculous!

[...]
>  void *kvmalloc_node(size_t size, gfp_t flags, int node)
>  {
>  	gfp_t kmalloc_flags = flags;
>  	void *ret;
>  
>  	/*
>  	 * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
>  	 * so the given set of flags has to be compatible.
>  	 */
>  	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
>  
>  	/*
>  	 * Make sure that larger requests are not too disruptive - no OOM
>  	 * killer and no allocation failure warnings as we have a fallback
>  	 */
> -	if (size > PAGE_SIZE)
> +	if (size > PAGE_SIZE) {
>  		kmalloc_flags |= __GFP_NORETRY | __GFP_NOWARN;
> +		kmalloc_flags &= ~__GFP_NOFAIL;
> +	}

No there are simply no users of this and even if had one which would be
legitimate it wouldn't be as simple as this. vmalloc _doesn't_ support
GFP_NOFAIL and it would be really non-trivial to implement it. If for
nothing else there are unconditional GFP_KERNEL allocations in some
vmalloc paths (which is btw. the reason why vmalloc is not GFP_NOFS
unsafe). It would take much more to add the non-failing semantic. And I
see _no_ reason to argue with this possibility when a) there is no such
user currently and b) it is even not clear whether we want to support
such a usecase.

[...]
> > > in http://lkml.kernel.org/r/20161218163727.GC8440@dhcp22.suse.cz .
> > > Indeed that trace is a __GFP_DIRECT_RECLAIM and it might not be blocking
> > > other workqueue items which a regular I/O depend on, I think there are
> > > !__GFP_DIRECT_RECLAIM memory allocation requests for issuing SCSI commands
> > > which could potentially start failing due to helping GFP_NOFS | __GFP_NOFAIL
> > > allocations with memory reserves. If a SCSI disk I/O request fails due to
> > > GFP_ATOMIC memory allocation failures because we allow a FS I/O request to
> > > use memory reserves, it adds a new problem.
> > 
> > Do you have any example of such a request? Anything that requires
> > a forward progress during IO should be using mempools otherwise it
> > is broken pretty much by design already. Also IO depending on NOFS
> > allocations sounds pretty much broken already. So I suspect the above
> > reasoning is just bogus.
> 
> You are missing my point. My question is "who needs memory reserves".
> I'm not saying that disk I/O depends on GFP_NOFS allocation. I'm worrying
> that [PATCH 3/3] consumes memory reserves when disk I/O also depends on
> memory reserves.
> 
> My understanding is that when accessing SCSI disks, SCSI protocol is used.
> SCSI driver allocates memory at runtime for using SCSI protocol using
> GFP_ATOMIC. And GFP_ATOMIC uses some of memory reserves. But [PATCH 3/3]
> also uses memory reserves. If memory reserves are consumed by [PATCH 3/3]
> to the level where GFP_ATOMIC cannot succeed, I think it causes troubles.

Yes and GFP_ATOMIC will have a deeper access to memory reserves than what
we are giving access to in patch 3. There is difference between
ALLOC_HARDER and ALLOC_HIGH. This is described in the changelog. Sure
GFP_NOFAIL will eat into part of the reserves which GFP_ATOMIC (aka
GFP_HIGH) could have used but a) this shouldn't happen unless we are
really getting out of memory and b) it should help other presumably
important allocations (why would they be GFP_NOFAIL otherwise right?).
So it is not just a free ticket to a scarce resource and IMHO it is
justified.

> I'm unable to obtain nice backtraces, but I think we can confirm that
> there are GFP_ATOMIC allocations (e.g. sg_alloc_table_chained() calls
> __sg_alloc_table(GFP_ATOMIC)) when we are using SCSI disks.

How are those blocking further progress? Failing atomic allocations are
nothing to lose sleep over. They cannot be, pretty by definition, relied
on to make a further progress.

[...]

I am _really_ getting tired of this discussion. You are making wrong or
unfounded claims again and again. I have no idea what are you trying to
achieve here but I simply do not see any sense in continuing in this
discussion.
-- 
Michal Hocko
SUSE Labs

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