[PATCH v2] mm/hugetlb: Restore failed global reservations to subpool

[Joshua Hahn <joshua.hahnjy@gmail.com>]

Commit a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
fixed an underflow error for hstate->resv_huge_pages caused by
incorrectly attributing globally requested pages to the subpool's
reservation.

Unfortunately, this fix also introduced the opposite problem, which would
leave spool->used_hpages elevated if the globally requested pages could
not be acquired. This is because while a subpool's reserve pages only
accounts for what is requested and allocated from the subpool, its
"used" counter keeps track of what is consumed in total, both from the
subpool and globally. Thus, we need to adjust spool->used_hpages in the
other direction, and make sure that globally requested pages are
uncharged from the subpool's used counter.

Each failed allocation attempt increments the used_hpages counter by
how many pages were requested from the global pool. Ultimately, this
renders the subpool unusable, as used_hpages approaches the max limit.

The issue can be reproduced as follows:
1. Allocate 4 hugetlb pages
2. Create a hugetlb mount with max=4, min=2
3. Consume 2 pages globally
4. Request 3 pages from the subpool (2 from subpool + 1 from global)
	4.1 hugepage_subpool_get_pages(spool, 3) succeeds.
		used_hpages += 3
	4.2 hugetlb_acct_memory(h, 1) fails: no global pages left
		used_hpages -= 2
5. Subpool now has used_hpages = 1, despite not being able to
   successfully allocate any hugepages. It believes it can now only
   allocate 3 more hugepages, not 4.

Repeating this process will ultimately render the subpool unable to
allocate any hugepages, since it believes that it is using the maximum
number of hugepages that the subpool has been allotted.

The underflow issue that the original commit fixes still remains fixed
as well.

Fixes: a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: stable@vger.kernel.org
---
v1 --> v2
- Moved "unsigned long flags" definition into the if statement it is used in
- Separated fix patch from cleanup patches for easier backporting for stable.

 mm/hugetlb.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5a147026633f..e48ff0c771f8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6713,6 +6713,15 @@ long hugetlb_reserve_pages(struct inode *inode,
 		 */
 		hugetlb_acct_memory(h, -gbl_resv);
 	}
+	/* Restore used_hpages for pages that failed global reservation */
+	if (gbl_reserve && spool) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&spool->lock, flags);
+		if (spool->max_hpages != -1)
+			spool->used_hpages -= gbl_reserve;
+		unlock_or_release_subpool(spool, flags);
+	}
 out_uncharge_cgroup:
 	hugetlb_cgroup_uncharge_cgroup_rsvd(hstate_index(h),
 					    chg * pages_per_huge_page(h), h_cg);

base-commit: c1a60bf0f6df5c8a6cb6840a0d2fb0e9caf9f7cc
-- 
2.47.3