Hi there,

are huge TLB pages always physically continous in memory?

What does follow_hugetlb_page do exactly? I simply don't
understand what the code does.

I would like to build up a simplified get_user_pages_sgl() to
build a scatter gather list from user space adresses.

If I want to coalesce physically continous pages (if they are
also virtually continious) anyway, can I write up a simplified
follow_hugetlb_page_sgl() function which handles the huge page
really as only one page?

Motivation:

Currently doing scatter gather DMA of user pages requires THREE
runs over the pages and I would like to save at least the second
one and possibly shorten the third one.

The three steps required:

   1) get_user_pages() to obtain the pages and lock them in page_cache
   2) translate the vector of pointers to struct page to a vector
      of struct scatterlist
   3) pci_map_sg() a decent amount[1], DMA it, wait for completion 
      or abortion, pci_unmap_sg() it and start again with the remainder

Step 2) could be eliminated completely and also the allocation of
the temporary vector of struct page.

Step 3) could be shortend, if I coalesce physically continous
ranges into a single scatterlist entry with just a ->length
bigger than PAGE_SIZE. I know that this is only worth it on
architectures, where physical address == bus address.

As each step is a for() loop and should be considered running on
more than 1MB worth of memory, I see significant improvements.

Without supporting huge TLB pages, I only add 700 bytes to the
kernel while simply copying get_user_pages() into a function,
which takes an vector of struct scatterlist instead of struct
page.

This sounds a promising tradeoff for a first time implementation.

Patch attached. No users yet, but they will follow. First
candidate is the v4l DMA stuff.

Regards

Ingo Oeser

[1] How much can I safely map on the strange architectures, where
   this is a limited? AFAIK there is no value or function telling
   me how far I can go.
-- 
Science is what we can tell a computer. Art is everything else. --- D.E.Knuth