On 03/02/2016 12:43 AM, Dan Williams wrote:
On Tue, Mar 1, 2016 at 12:31 AM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
On 03/01/2016 03:06 AM, Dan Williams wrote:
On Mon, Feb 29, 2016 at 4:06 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
On 29.2.2016 18:55, Dan Williams wrote:
On Mon, Feb 29, 2016 at 4:33 AM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
On 02/02/2016 06:42 AM, Andrew Morton wrote:
In this case it's already part of the equation because:
config ZONE_DEVICE
depends on MEMORY_HOTPLUG
depends on MEMORY_HOTREMOVE
...and those in turn depend on SPARSEMEM.
Fine, but then SPARSEMEM_VMEMMAP should be still an available subvariant
of
SPARSEMEM with SECTION_WIDTH=0.
It should be, but not for the ZONE_DEVICE case. ZONE_DEVICE depends
on x86_64 which means ZONE_DEVICE also implies SPARSEMEM_VMEMMAP
since:
config ARCH_SPARSEMEM_ENABLE
def_bool y
depends on X86_64 || NUMA || X86_32 || X86_32_NON_STANDARD
select SPARSEMEM_STATIC if X86_32
select SPARSEMEM_VMEMMAP_ENABLE if X86_64
Now, if a future patch wants to reclaim page flags space for other
usages outside of ZONE_DEVICE it can do the work to handle the
SPARSEMEM_VMEMMAP=n case. I don't see a reason to fold that
distinction into the current patch given the current constraints.
OK so that IUUC shows that x86_64 should be always fine without decreasing
the range for NODES_SHIFT? That's basically my point - since there's a
configuration where things don't fit (32bit?), the patch broadly decreases
range for NODES_SHIFT for everyone, right?
So I went hunting for the x86_64 config that sent me off in this
direction in the first place, but I can't reproduce it. I'm indeed
able to fit ZONE_DEVICE + ZONE_DMA + NODES_SHIFT(10) without
overflowing page flags. Maybe we reduced some usage page->flags usage
between 4.3 and 4.5 and I missed it?
Oh, I think I see it now. SPARSEMEM_VMEMMAP_ENABLE only *allows to
enable* CONFIG_SPARSEMEM_VMEMMAP, it doesn't force it:
config SPARSEMEM_VMEMMAP
bool "Sparse Memory virtual memmap"
depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
default y
In any event, you're right we can indeed fit ZONE_DEVICE into the
current MAXSMP definition. I'll respin the patch.
But I still believe that that your respin is better than this variant.
We shouldn't broadly limit the range in one of the options, when there
are multiple options affecting the usage of bits. There's a warning if
the overal configuration is "too large", which could potentially be more
detailed. But we never said configuring the kernel is trivial ;-)
Also in this case the "default y" for SPARSEMEM_VMEMMAP should prevent
surprise when one enables ZONE_DEVICE through nvdimm and doesn't fiddle
with the lowlevel details. As long as it takes multiple explicit choices
differing from defaults to get to the warning, I'd say we are fine.
Thanks for probing on this!
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