panic: kmem_malloc(16384): kmem_map too small: md-mounted /tmp filled up

[Scott Long]

Kostik Belousov wrote:
> On Mon, Mar 05, 2007 at 10:17:14PM +0300, Yar Tikhiy wrote:
>> On Mon, Mar 05, 2007 at 01:14:29PM -0500, Mikhail Teterin wrote:
>>> On Monday 05 March 2007 08:23, Yar Tikhiy wrote:
>>> = > How will it break them?  swap backing only touches swap if there is
>>> = > memory pressure, i.e. precisely the situation in which malloc backing
>>> = > will panic.
>>> = 
>>> = I forgot that in BSD swap wouldn't be allocated in advance to its
>>> = consumers.  Then removing the -M flag and making swap backing the
>>> = default is a very sound choice.  Thank you for correcting me.
>>>
>>> Yar, would you change the man-page's advice and the default, then?
>> Yes, I'll be glad to if no objections arise until I finish updating
>> my CURRENT machine, i.e., tomorrow. :-)
>>
>>> Someone still needs to look into the panic... Who would that be?
>> Obviously, Mr(s). Someone. :-)
>>
>> The md case exposes a quite tangled nature of the problem.  Funnily
>> enough, kernel malloc() cannot just fail in the case because it
>> must not fail if called with M_WAITOK.  This means that the system
>> has quite a rough choice:
>>
>> - put the requesting thread to sleep forever;
>> - grow kmem_map, eventually sacrifice all RAM to the greedy thread
>>   and die sooner or later;
>> - panic immediately.
>>
>> If all malloc() callers in the kernel were ready to deal with
>> allocation failure, the system could just tell the greedy thread
>> to buzz off.  But too many kernel parts depend on malloc(M_WAITOK)
>> never failing.  Perhaps it's the root of the problem.
> 
> Mark callers that are ready for M_WAITOK failure with some additional
> flag, like M_FAILOK (feel free to propose meaningful name there).
> At least malloc()-based md could then use it.

The panic is a chronic problem that really needs to be fixed in general.
However, the md code should probably be modified to reject any 
malloc-backed size larger than some trivial (and arbitrary) value, like 
say 1MB.  It's really inferior to being swap-backed, and it only 
encourages foot-shooting and these unclear panics.

Scott