On Thu, Jun 30, 2011 at 04:20:13PM +0200, Igor Mammedov
wrote:> On 06/29/2011 11:53 AM, Igor Mammedov wrote:
> >Hi,
> >
> >I have a question about how upstream patches:
> >
> >093d7b4639951 xen: release unused free memory
> >f89e048e76da7 xen: make sure pages are really part of domain
> >before freeing
> >
> >are supposed to work. Was it intended for freeing unused memory if
> >we boot
> >kernel with mem=XX option and domU is configured with more memory
> >than XX?
> >
> >
>
> In regard to RHBZ#523130, I made additional debugging after applying
> above mentioned patches. Result: unsed memory is not freed if we pass
> mex=XXXM option to pv guest kernel.
>
> Here is call order:
> ===========> setup_arch ->
> -> setup_memory_map
> -> x86_init.resources.memory_setup == xen_memory_setup
> before xen_return_unused_memory call we have e820 map like this
> that contains all memory provide by xen:
> BIOS-provided physical RAM map:
> Xen: 0000000000000000 - 00000000000a0000 (usable)
> Xen: 00000000000a0000 - 0000000000100000 (reserved)
> Xen: 0000000000100000 - 0000000200000000 (usable
> * -> xen_return_unused_memory
>
> ** -> parse_early_param
> ============>
> *
> code fragment:
> @xen_return_unused_memory
> for (i = 0; i< e820->nr_map&& last_end<
max_addr; i++) {
> phys_addr_t end = e820->map[i].addr;
> end = min(max_addr, end);
>
> released += xen_release_chunk(last_end, end);
> last_end = e820->map[i].addr + e820->map[i].size;
> }
> in best case is nop since xen_release_chunk will not do anything if
end<> last_end and with our example map pairs (last_end, end) will look
like:
> last_end end
> 0x0 0x0
> 0xa0000 0xa0000
> 0x100000 0x100000
> i.e. last_end == end => xen_release_chunk is nop
> in worst case xen_release_chunk may try to release not existed gap if e820
is
> sparse.
Those are OK for memory below that region. We actually don''t want to
free it
as the PV guest stops booting.>
> last fragment of xen_return_unused_memory
> if (last_end< max_addr)
> released += xen_release_chunk(last_end, max_addr);
>
> has sense only if
> last_end(e820 val)< max_addr (max hv provided addr)
> and that could be only if "mem=" parameter was applied to e820
map.
Ah.> However ** parse_early_param is called after
x86_init.resources.memory_setup in
> setup_memory_map. So patches:
> 093d7b4639951 xen: release unused free memory
> f89e048e76da7 xen: make sure pages are really part of domain before freeing
> are effectively nop code now.
>
> Problem exist not only in rhel6 code but fc15 as well and I guess upstream
> is affected to.
well, they do _some_ freeing. I see ''released XX pages'' quite
often on my boxes.
But yes, this a problem - we allocate more memory than we need. Do you have any
patches in mind?
>
>
> --
> Thanks,
> Igor
>
>
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