Re: [RFC] vmalloc: add warning in __vmalloc

[Nick Piggin]

On 27 April 2012 20:36, David Rientjes <rientjes@xxxxxxxxxx> wrote:
> On Fri, 27 Apr 2012, Minchan Kim wrote:
>
>> Now there are several places to use __vmalloc with GFP_ATOMIC,
>> GFP_NOIO, GFP_NOFS but unfortunately __vmalloc calls map_vm_area
>> which calls alloc_pages with GFP_KERNEL to allocate page tables.
>> It means it's possible to happen deadlock.
>> I don't know why it doesn't have reported until now.
>>
>> Firstly, I tried passing gfp_t to lower functions to support __vmalloc
>> with such flags but other mm guys don't want and decided that
>> all of caller should be fixed.
>>
>> http://marc.info/?l=linux-kernel&m=133517143616544&w=2
>>
>> To begin with, let's listen other's opinion whether they can fix it
>> by other approach without calling __vmalloc with such flags.
>>
>> So this patch adds warning to detect and to be fixed hopely.
>> I Cced related maintainers.
>> If I miss someone, please Cced them.
>>
>> side-note:
>>   I added WARN_ON instead of WARN_ONCE to detect all of callers
>>   and each WARN_ON for each flag to detect to use any flag easily.
>>   After we fix all of caller or reduce such caller, we can merge
>>   a warning with WARN_ONCE.
>>
>
> I disagree with this approach since it's going to violently spam an
> innocent kernel user's log with no ratelimiting and for a situation that
> actually may not be problematic.

With WARN_ON_ONCE, it should be good.

>
> Passing any of these bits (the difference between GFP_KERNEL and
> GFP_ATOMIC) only means anything when we're going to do reclaim.  And I'm
> suspecting we would have seen problems with this already since
> pte_alloc_kernel() does __GFP_REPEAT on most architectures meaning that it
> will loop infinitely in the page allocator until at least one page is
> freed (since its an order-0 allocation) which would hardly ever happen if
> __GFP_FS or __GFP_IO actually meant something in this context.
>
> In other words, we would already have seen these deadlocks and it would
> have been diagnosed as a vmalloc(GFP_ATOMIC) problem.  Where are those bug
> reports?

That's not sound logic to disprove a bug.

I think simply most callers are permissive and don't mask out flags.
But for example a filesystem holding an fs lock and then doing
vmalloc(GFP_NOFS) can certainly deadlock.
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