Re: [PATCH 07/11] x86, memblock: Set lowest limit for memblock_alloc_base_nid().

[Toshi Kani <toshi.kani@hp.com>]

On Wed, 2013-09-04 at 10:05 +0800, Tang Chen wrote:
> On 09/04/2013 08:37 AM, Toshi Kani wrote:
> > On Tue, 2013-08-27 at 17:37 +0800, Tang Chen wrote:
> >> memblock_alloc_base_nid() is a common API of memblock. And it calls
> >> memblock_find_in_range_node() with %start = 0, which means it has no
> >> limit for the lowest address by default.
> >>
> >> 	memblock_find_in_range_node(0, max_addr, size, align, nid);
> >>
> >> Since we introduced current_limit_low to memblock, if we have no limit
> >> for the lowest address or we are not sure, we should pass
> >> MEMBLOCK_ALLOC_ACCESSIBLE to %start so that it will be limited by the
> >> default low limit.
> >>
> >> dma_contiguous_reserve() and setup_log_buf() will eventually call
> >> memblock_alloc_base_nid() to allocate memory. So if the allocation order
> >> is from low to high, they will allocate memory from the lowest limit
> >> to higher memory.
> >
> > This requires the callers to use MEMBLOCK_ALLOC_ACCESSIBLE instead of 0.
> > Is there a good way to make sure that all callers will follow this rule
> > going forward?  Perhaps, memblock_find_in_range_node() should emit some
> > message if 0 is passed when current_order is low to high and the boot
> > option is specified?
> 
> How about set this as the default rule:
> 
> 	When using from low to high order, always allocate memory from
> 	current_limit_low.
> 
> So far, I think only movablenode boot option will use this order.

Sounds good to me.

> > Similarly, I wonder if we should have a check to the allocation size to
> > make sure that all allocations will stay small in this case.
> >
> 
> We can check the size. But what is the stragety after we found that the 
> size
> is too large ?  Do we refuse to allocate memory ?  I don't think so.

We can just add a log message.  No need to fail.

> I think only relocate_initrd() and reserve_crachkernel() could allocate 
> large
> memory. reserve_crachkernel() is easy to reorder, but reordering 
> relocate_initrd()
> is difficult because acpi_initrd_override() need to access to it with va.
> 
> I think on most servers, we don't need to do relocate_initrd(). initrd 
> will be
> loaded to mapped memory in normal situation. Can we just leave it there ?

Since this approach relies on the assumption that all allocations are
small enough, it would be nice to have a way to verify if it remains
true.  How about we measure a total amount of allocations while the
order is low to high, and log it when switched to high to low?  This
way, we can easily monitor the usage.

Thanks,
-Toshi

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