get_user_pages() still broken in 2.6

Timur Tabi
2004-09-28T22:40:26Z

I was hoping that this bug would be fixed in the 2.6 kernels, but
apparently it hasn't been.

Function get_user_pages() is supposed to lock user memory. However,
under extreme memory constraints, the kernel will swap out the "locked"
memory.

I have a test app which does this:

1) Calls our driver, which issues a get_user_pages() call for one page.
2) Calls our driver again to get the physical address of that page (the
driver uses pgd/pmd/pte_offset).
3) Tries allocate 1GB of memory (this system has 1GB of physical RAM).
4) Tries to get the physical address again.

In step 4, the physical address is usually zero, which means either
pgd_offset or pmd_offset failed. This indicates the page was swapped out.

I don't understand how this bug can continue to exist after all this
time. get_user_pages() is supposed to lock the memory, because drivers
use it for DMA'ing directly into user memory.

--
Timur Tabi
Staff Software Engineer
timur.tabi@ammasso.com