[PATCHv5 2/8] zsmalloc: add documentation

[Seth Jennings]

On 02/16/2013 12:21 AM, Ric Mason wrote:
> On 02/14/2013 02:38 AM, Seth Jennings wrote:
>> This patch adds a documentation file for zsmalloc at
>> Documentation/vm/zsmalloc.txt
>>
>> Signed-off-by: Seth Jennings <sjenning at linux.vnet.ibm.com>
>> ---
>>   Documentation/vm/zsmalloc.txt |   68
>> +++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 68 insertions(+)
>>   create mode 100644 Documentation/vm/zsmalloc.txt
>>
>> diff --git a/Documentation/vm/zsmalloc.txt
>> b/Documentation/vm/zsmalloc.txt
>> new file mode 100644
>> index 0000000..85aa617
>> --- /dev/null
>> +++ b/Documentation/vm/zsmalloc.txt
>> @@ -0,0 +1,68 @@
>> +zsmalloc Memory Allocator
>> +
>> +Overview
>> +
>> +zmalloc a new slab-based memory allocator,
>> +zsmalloc, for storing compressed pages.  It is designed for
>> +low fragmentation and high allocation success rate on
>> +large object, but <= PAGE_SIZE allocations.
>> +
>> +zsmalloc differs from the kernel slab allocator in two primary
>> +ways to achieve these design goals.
>> +
>> +zsmalloc never requires high order page allocations to back
>> +slabs, or "size classes" in zsmalloc terms. Instead it allows
>> +multiple single-order pages to be stitched together into a
>> +"zspage" which backs the slab.  This allows for higher allocation
>> +success rate under memory pressure.
>> +
>> +Also, zsmalloc allows objects to span page boundaries within the
>> +zspage.  This allows for lower fragmentation than could be had
>> +with the kernel slab allocator for objects between PAGE_SIZE/2
>> +and PAGE_SIZE.  With the kernel slab allocator, if a page compresses
>> +to 60% of it original size, the memory savings gained through
>> +compression is lost in fragmentation because another object of
>> +the same size can't be stored in the leftover space.
>> +
>> +This ability to span pages results in zsmalloc allocations not being
>> +directly addressable by the user.  The user is given an
>> +non-dereferencable handle in response to an allocation request.
>> +That handle must be mapped, using zs_map_object(), which returns
>> +a pointer to the mapped region that can be used.  The mapping is
>> +necessary since the object data may reside in two different
>> +noncontigious pages.
> 
> Do you mean the reason of  to use a zsmalloc object must map after
> malloc is object data maybe reside in two different nocontiguous pages?

Yes, that is one reason for the mapping.  The other reason (more of an
added bonus) is below.

> 
>> +
>> +For 32-bit systems, zsmalloc has the added benefit of being
>> +able to back slabs with HIGHMEM pages, something not possible
> 
> What's the meaning of "back slabs with HIGHMEM pages"?

By HIGHMEM, I'm referring to the HIGHMEM memory zone on 32-bit systems
with larger that 1GB (actually a little less) of RAM.  The upper 3GB
of the 4GB address space, depending on kernel build options, is not
directly addressable by the kernel, but can be mapped into the kernel
address space with functions like kmap() or kmap_atomic().

These pages can't be used by slab/slub because they are not
continuously mapped into the kernel address space.  However, since
zsmalloc requires a mapping anyway to handle objects that span
non-contiguous page boundaries, we do the kernel mapping as part of
the process.

So zspages, the conceptual slab in zsmalloc backed by single-order
pages can include pages from the HIGHMEM zone as well.

Seth