Re: [RFC] manage multiple entries of scratch page with scatterlist

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jun 12, 2014 at 12:49:47AM +0100, Siluvery, Arun wrote:
> Hi,
> 
> I am working on a feature to implement support for gem objects to have
> variable size and realized a problem with the current implementation.
> Please advice me how to handle this situation efficiently.
> 
> In this implementation the backing store of the object is replaced with
> scratch pages according to input range; Initially I store table entries in
> an array, replace relevant entries with scratch pages and I am using
> sg_alloc_table_from_pages() to create new sg_table which is assigned to the
> object. This implementation works as expected but I realized it is wasting
> memory as scratch page count increases.
> 
> Consider the worst case scenario where all pages are replaced with scratch
> pages.
> 
> The fn sg_alloc_table_from_pages() first computes the number of chunks based
> on the page frame numbers. PFNs that are consecutive form a chunk and it
> allocates scatterlists for each chunk which form the sg_table.
> 
> In case of scratch pages they get the same pfn for each page and
> sg_alloc_table_from_pages() considers them not part of a chunk and it
> allocates scatterlist structure for each scratch page which takes lot of
> memory as the object size increases.
> 
> I have to tried to modify sg_alloc_table_from_pages() implementation to
> check for scratch pfn and consider them as single chunk but after the update
> when iterating through for_each_sg_page() I am seeing different page
> addresses instead of all pointing to scratch page.
> 
> Eg. In an object of size 8 pages, scratch_page = ffffea0001120000 and pfn:
> 0x00044800, the result I get is,
> 
> page[0]: ffffea0001120000, pfn: 0x00044800,
> page[1]: ffffea0001120040, pfn: 0x00044801,
> page[2]: ffffea0001120080, pfn: 0x00044802,
> page[3]: ffffea00011200c0, pfn: 0x00044803,
> page[4]: ffffea0001120100, pfn: 0x00044804,
> page[5]: ffffea0001120140, pfn: 0x00044805,
> page[6]: ffffea0001120180, pfn: 0x00044806,
> page[7]: ffffea00011201c0, pfn: 0x00044807,
> 
> How to manage multiple pages that have same pfn with a single scatterlist
> and still have it's length equal to (PAGE_SIZE*chunk_size)?
> 
> I would really appreciate any suggestions to improve this implementation.

sg tables don't have the idea of repeating a given page, since it doesn't
make a lot of sense. Is the memory overhead really a big problem?

Extending the sg implementation with a flag somewhere to repeat a given
page instead of incrementing might be possible. But will be a bit of
effort to push that through the process since we'll touch code outside of
drm.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch