Re: [Xen-devel] Xen 4.0.1 "xc_map_foreign_batch: mmap failed: Cannot allocate memory"

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 07.01.11 at 12:18, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
wrote:
> On Fri, 7 Jan 2011, Jan Beulich wrote:
>> >>> On 06.01.11 at 21:49, Charles Arnold wrote:
>> > >>> On 1/6/2011 at 10:14 AM, in message <4D25C782.5B74.0091.0@xxxxxxxxxx>, 
> Charles Arnold wrote: 
>> > Attached is the messages file with the printk output.
>> 
>> Hmm, a failure due to may_expand_vm() is really odd. Something
>> must be explicitly setting a non-infinite RLIMIT_AS on qemu-dm (or
>> one of its parents), as the default is "infinite" (as reaching "infinity"
>> - being ~0UL - is simply impossible, and unduly large lengths should
>> be caught by get_unmapped_area() already).
>> 
>> /proc/<pid>/limits would at least tell us what the limit is.
>> 
> 
> Knowing this would be very interesting.

I just found that on SLE11 this gets set to 80% (admin controllable)
of the sum of physical (not accounting for the balloon) and swap
memory. That's (at least on large systems using a relatively low
dom0_mem= value) likely awfully low for qemu-dm serving large
guests.

However, it's only rlim_cur that gets set this low by default, and
hence it would seem reasonable to me to have qemu-dm bump it
to whatever getrlimit() returns in rlimit_max.

>> And certainly qemu-dm needs to be prepared to have a
>> non-infinite address space limit set on it.
>> 
> 
> Currently the number of buckets and the bucket size in the mapcache are
> statically defined depending on x86_32/x86_64.
> It shouldn't be difficult to make them dynamic depending on RLIMIT_AS.

That still wouldn't help if RLIMIT_AS gets changed when it's already
running. The only proper way to deal with the situation as a whole
(including but not limited to rlim_max being relatively low) is to get
proper error handling implemented, either causing guest I/O to be
throttled when mmap() fails, or no longer used mappings cleared
(if that isn't being done already).

Jan


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