x86/mm: Drop unnecessary PMD page copy when freeing

In the Linux kernel, the following vulnerability has been resolved:

x86/mm: Drop unnecessary PMD page copy when freeing

On a box with a discrete GPU, lockdep reports a possible deadlock as
soon as kswapd shrinks the TTM page pool. The immediate cause is an
x86 commit that added an mmap_read_lock() to kernel page protection
munging code.

The huge vmap code holds the same lock over a GFP_KERNEL allocation,
which is a no-no now that reclaim can take it. That allocation is in a
page table *free* path and ends up being for dubious purposes[1].
Basically, it tries to avoid hardware setting Accessed=1 in page table
entries that are unreachable by the hardware, a non-issue.

Remove the PMD copy. Detach the original PMD page at the PUD, flush
the mid-level caches, and free the PTE tables straight from the
detached PMD page. With no allocation left, the locking issue is gone.

Lockdep splat/analysis:

WARNING: possible circular locking dependency detected
7.3.0-rc3-f6e7b42bf05b+ #183 Tainted: G U
------------------------------------------------------
kswapd0/269 is trying to acquire lock:
((init_mm).mmap_lock){++++}-{4:4}, at: change_page_attr_set_clr+0x29a/0x4a0
but task is already holding lock:
(pool_shrink_rwsem){.+.+}-{4:4}, at: ttm_pool_shrink+0xb2/0x330 [ttm]
Chain exists of:
(init_mm).mmap_lock --> fs_reclaim --> pool_shrink_rwsem

The cycle is built from three edges:

1) pool_shrink_rwsem -> (init_mm).mmap_lock

The TTM shrinker restores the caching attribute of every page it
frees, while holding pool_shrink_rwsem:

ttm_pool_shrink()
-> ttm_pool_dispose_list()
-> ttm_pool_free_page()
-> set_pages_wb()
-> change_page_attr_set_clr() [ init_mm mmap read lock ]

2) fs_reclaim -> pool_shrink_rwsem

The same shrinker, called from reclaim.

3) (init_mm).mmap_lock -> fs_reclaim

ioremap() installing a huge PUD mapping over an existing PMD table:

ioremap_page_range()
-> vmap_range_noflush()
-> vmap_try_huge_pud() [ init_mm mmap read lock ]
-> pud_free_pmd_page()
-> __get_free_page(GFP_KERNEL) [ enters reclaim ]

[ dhansen: Lots of changelog munging/trimming and merged comments from my
version of the fix. ]