Patch series "mm/hugetlb: Restore the reservation", v2.

This is a fix for a case where a backing huge page could stolen after
madvise(MADV_DONTNEED).

A full reproducer is in selftest. See
https://lore.kernel.org/all/20240105155419.1939484-1-leitao@debian.org/

In order to test this patch, I instrumented the kernel with LOCKDEP and
KASAN, and run the following tests, without any regression:
  * The self test that reproduces the problem
  * All mm hugetlb selftests
	SUMMARY: PASS=9 SKIP=0 FAIL=0
  * All libhugetlbfs tests
	PASS:     0     86
	FAIL:     0      0


This patch (of 2):

Currently there is a bug that a huge page could be stolen, and when the
original owner tries to fault in it, it causes a page fault.

You can achieve that by:
  1) Creating a single page
	echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

  2) mmap() the page above with MAP_HUGETLB into (void *ptr1).
	* This will mark the page as reserved
  3) touch the page, which causes a page fault and allocates the page
	* This will move the page out of the free list.
	* It will also unreserved the page, since there is no more free
	  page
  4) madvise(MADV_DONTNEED) the page
	* This will free the page, but not mark it as reserved.
  5) Allocate a secondary page with mmap(MAP_HUGETLB) into (void *ptr2).
	* it should fail, but, since there is no more available page.
	* But, since the page above is not reserved, this mmap() succeed.
  6) Faulting at ptr1 will cause a SIGBUS
	* it will try to allocate a huge page, but there is none
	  available

A full reproducer is in selftest. See
https://lore.kernel.org/all/20240105155419.1939484-1-leitao@debian.org/

Fix this by restoring the reserved page if necessary.

These are the condition for the page restore:

 * The system is not using surplus pages. The goal is to reduce the
   surplus usage for this case.
 * If the VMA has the HPAGE_RESV_OWNER flag set, and is PRIVATE. This is
   safely checked using __vma_private_lock()
 * The page is anonymous

Once this is scenario is found, set the `hugetlb_restore_reserve` bit in
the folio. Then check if the resv reservations need to be adjusted
later, done later, after the spinlock, since the vma_xxxx_reservation()
might touch the file system lock.

Link: https://lkml.kernel.org/r/20240205191843.4009640-1-leitao@debian.org
Link: https://lkml.kernel.org/r/20240205191843.4009640-2-leitao@debian.orgSigned-off-by: Breno Leitao <leitao@debian.org>
Suggested-by: Rik van Riel <riel@surriel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>