On Sun, 2021-03-21 at 19:45 +0100, Thomas Hellström (Intel) wrote:
TTM sets up huge page-table-entries both to system- and device memory, and we don't want gup to assume there are always valid backing struct pages for these. For PTEs this is handled by setting the pte_special bit, but for the huge PUDs and PMDs, we have neither pmd_special nor pud_special. Normally, huge TTM entries are identified by looking at vma_is_special_huge(), but fast gup can't do that, so as an alternative define _devmap entries for which there are no backing dev_pagemap as special, update documentation and make huge TTM entries _devmap, after verifying that there is no backing dev_pagemap.
Please do not abuse p{m,u}d_devmap like this. I'm in the process of removing get_devpagemap() from the gup-fast path [1]. Instead there should be space for p{m,u}d_special in the page table entries (at least for x86-64). So the fix is to remove that old assumption that huge pages can never be special.
[1]: http://lore.kernel.org/r/161604050866.1463742.7759521510383551055.stgit@dwil...