Re: [PATCH 1/2] mm: Allow small allocations to fail

[Michal Hocko]

On Thu 12-03-15 21:54:47, Tetsuo Handa wrote:
> (The Cc: line seems to be partially truncated. Please re-add if needed.)
> 
> Michal Hocko wrote:
> > Finally, if a non-failing allocation is unavoidable then __GFP_NOFAIL
> > flag is there to express this strong requirement. It is much better to
> > have a simple way to check all those places and come up with a solution
> > which will guarantee a forward progress for them.
> 
> Keeping gfp flags passed to ongoing allocation inside "struct task_struct"
> will allow the OOM killer to skip OOM victims doing __GFP_NOFAIL.
> http://marc.info/?l=linux-mm&m=141671829611143&w=2 would give a hint.

Not related to the patch IMNHO. This patch is about allocations which
might fail. The comment simply says that those who _cannot_ should
be annotated properly.
 
> > As this behavior is established for many years we cannot change it
> > immediately. This patch instead exports a new sysctl/proc knob which
> > tells allocator how much to retry. The higher the number the longer will
> > the allocator loop and try to trigger OOM killer when the memory is too
> > low. This implementation counts only those retries which involved OOM
> > killer because we do not want to be too eager to fail the request.
> 
> I prefer jiffies timeouts than retry counts, for jiffies will allow vmcore
> to tell how long the process was stalled for memory allocation.
> http://marc.info/?l=linux-mm&m=141671821111135&w=1 and
> http://marc.info/?l=linux-mm&m=141709978209207&w=1 would give a hint.
> 
> > The default value is ULONG_MAX which basically preserves the current
> > behavior (endless retries). The idea is that we start with testing
> > systems first and lower the value to catch potential fallouts (crashes
> > due to unchecked failures or other misbehavior like FS ro-remounts
> > etc...). Allocation failures are already reported by warn_alloc_failed
> > so we should be able to catch the allocation path before an issue is
> > triggered.
> 
> Few developers are using fault-injection capability (CONFIG_FAILSLAB and
> CONFIG_FAIL_PAGE_ALLOC). Even less developers would be performing OOM
> stress tests. Printing allocation failure messages only upon OOM condition
> is Whack-A-Mole where moles remain hidden until distribution kernel users
> by chance (or by intent) triggered OOM condition.
> 
> I tried SystemTap-based mandatory fault-injection hooks at
> http://marc.info/?l=linux-kernel&m=141951300713051&w=2 and I reported
> random crashes at
> http://lists.freedesktop.org/archives/dri-devel/2015-January/075922.html .
> How can we find the exact culprit allocation when an issue is triggered
> some time after the first failure messages?
> 
> I think that your knob helps avoiding infinite loop if lower value is
> given, but I don't think that your knob helps catching potential fallouts.

An allocation failure with the full trace will at least help you see
_who_ might be causing troubles later. Some bugs might be subtle and
harder to debug, of course.
 
> > We will try to encourage distributions to change the default in the
> > second step so that we get a much bigger exposure.
> 
> Can we expect that distribution kernel users are willing to perform OOM
> stress tests which kernel developers did not perform?
> 
> > And finally we can change the default in the kernel while still keeping
> > the knob for conservative configurations. This will be long run but
> > let's start.
> 
> And finally what patches will you propose for already running systems
> using distribution kernels? I can't wait for years (or decades) until
> your knob and fixes for fallouts are backported.

I am afraid I do not care about old and distribution kernels in _this_
_proposal_. This is an attempt to move away from a concept which is not
healthy IMHO and it will take long time to get make it happen. OOM
killer deadlocks is just one of the reasons. So please do not conflate
those two things together.

-- 
Michal Hocko
SUSE Labs
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