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Date:	Mon, 30 Apr 2012 10:52:30 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	npiggin@...il.com, kamezawa.hiroyu@...fujitsu.com,
	kosaki.motohiro@...il.com, rientjes@...gle.com,
	Neil Brown <neilb@...e.de>,
	Artem Bityutskiy <dedekind1@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Theodore Ts'o <tytso@....edu>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	James Morris <jmorris@...ei.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Sage Weil <sage@...dream.net>
Subject: Re: [RFC] vmalloc: add warning in __vmalloc

On 04/28/2012 07:00 AM, Andrew Morton wrote:

> On Fri, 27 Apr 2012 17:42:24 +0900
> Minchan Kim <minchan@...nel.org> 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.
> 
> Just WARN_ONCE, please.  If that exposes some sort of calamity then we
> can reconsider.


NP.

> 
>>
>> ...
>>
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -1700,6 +1700,15 @@ static void *__vmalloc_node(unsigned long size, unsigned long align,
>>  			    gfp_t gfp_mask, pgprot_t prot,
>>  			    int node, void *caller)
>>  {
>> +	/*
>> +	 * This function calls map_vm_area so that it allocates
>> +	 * page table with GFP_KERNEL so caller should avoid using
>> +	 * GFP_NOIO, GFP_NOFS and !__GFP_WAIT.
>> +	 */
>> +	WARN_ON(!(gfp_mask & __GFP_WAIT));
>> +	WARN_ON(!(gfp_mask & __GFP_IO));
>> +	WARN_ON(!(gfp_mask & __GFP_FS));
>> +
>>  	return __vmalloc_node_range(size, align, VMALLOC_START, VMALLOC_END,
>>  				gfp_mask, prot, node, caller);
>>  }
> 
> This seems strange.  There are many entry points to this code and the
> patch appears to go into a randomly-chosen middle point in the various
> call chains and sticks a check in there.  Why was __vmalloc_node()
> chosen?  Does this provide full coverage or all entry points?


I think it covers all of caller with calls __vmalloc with gfp_flags.
Only exception is __vmalloc_node_range which is called by module_alloc
but it surely calls __vmalloc_node_range with GFP_KERNEL so it's no problem now.
If you want to catch potential use of __vmalloc_node_range in future, I can move it
to it. 

> 
> 
> 
> Also, the patch won't warn in the most problematic cases such as
> vmalloc() being called from a __GFP_NOFS context.  Presumably there are


I agree but this patch's goal is just to prevent calling __vmalloc with GFP_ATOMIC, GFP_NOFS and
GFP_NOIO. We should consider vmalloc on __GFP_NOFS context as another problem and maybe
reclaimfs lockdep would be a good start point.

> might_sleep() warnings somewhere on the allocation path which will

> catch vmalloc() being called from atomic contexts.


Yes.

> 
> I'm not sure what to do about that - we don't have machinery in place
> to be able to detect when a GFP_KERNEL allocation is deadlockable. 
> Perhaps a lot of hacking on lockdep might get us this - we'd need to
> teach lockdep about which locks prohibit FS entry, which locks prevent
> IO entry, etc.  And there are secret locks such as ext3/4
> journal_start(), and bitlocks and lock_page().  eek.

> 

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-- 
Kind regards,
Minchan Kim
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