[PATCH V2] mm/hugetlb resv map memory leak for placeholder entries

[Mike Kravetz <mike.kravetz@oracle.com>]

Dmitry Vyukov reported the following memory leak

unreferenced object 0xffff88002eaafd88 (size 32):
  comm "a.out", pid 5063, jiffies 4295774645 (age 15.810s)
  hex dump (first 32 bytes):
    28 e9 4e 63 00 88 ff ff 28 e9 4e 63 00 88 ff ff  (.Nc....(.Nc....
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<     inline     >] kmalloc include/linux/slab.h:458
    [<ffffffff815efa64>] region_chg+0x2d4/0x6b0 mm/hugetlb.c:398
    [<ffffffff815f0c63>] __vma_reservation_common+0x2c3/0x390 mm/hugetlb.c:1791
    [<     inline     >] vma_needs_reservation mm/hugetlb.c:1813
    [<ffffffff815f658e>] alloc_huge_page+0x19e/0xc70 mm/hugetlb.c:1845
    [<     inline     >] hugetlb_no_page mm/hugetlb.c:3543
    [<ffffffff815fc561>] hugetlb_fault+0x7a1/0x1250 mm/hugetlb.c:3717
    [<ffffffff815fd349>] follow_hugetlb_page+0x339/0xc70 mm/hugetlb.c:3880
    [<ffffffff815a2bb2>] __get_user_pages+0x542/0xf30 mm/gup.c:497
    [<ffffffff815a400e>] populate_vma_page_range+0xde/0x110 mm/gup.c:919
    [<ffffffff815a4207>] __mm_populate+0x1c7/0x310 mm/gup.c:969
    [<ffffffff815b74f1>] do_mlock+0x291/0x360 mm/mlock.c:637
    [<     inline     >] SYSC_mlock2 mm/mlock.c:658
    [<ffffffff815b7a4b>] SyS_mlock2+0x4b/0x70 mm/mlock.c:648

Dmitry identified a potential memory leak in the routine region_chg,
where a region descriptor is not free'ed on an error path.

However, the root cause for the above memory leak resides in region_del.
In this specific case, a "placeholder" entry is created in region_chg.  The
associated page allocation fails, and the placeholder entry is left in the
reserve map.  This is "by design" as the entry should be deleted when the
map is released.  The bug is in the region_del routine which is used to
delete entries within a specific range (and when the map is released).
region_del did not handle the case where a placeholder entry exactly matched
the start of the range range to be deleted.  In this case, the entry would
not be deleted and leaked.  The fix is to take these special placeholder
entries into account in region_del.

The region_chg error path leak is also fixed.

V2: The original version of the patch did not correctly handle placeholder
    entries before the range to be deleted.  The new check is more specific
    and only matches placeholders at the start of range.

Fixes: feba16e25a57 ("add region_del() to delete a specific range of entries")
Cc: stable@vger.kernel.org [4.3]
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
---
 mm/hugetlb.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1101ccd94..c895ab9 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -372,8 +372,10 @@ retry_locked:
 		spin_unlock(&resv->lock);
 
 		trg = kmalloc(sizeof(*trg), GFP_KERNEL);
-		if (!trg)
+		if (!trg) {
+			kfree(nrg);
 			return -ENOMEM;
+		}
 
 		spin_lock(&resv->lock);
 		list_add(&trg->link, &resv->region_cache);
@@ -483,8 +485,16 @@ static long region_del(struct resv_map *resv, long f, long t)
 retry:
 	spin_lock(&resv->lock);
 	list_for_each_entry_safe(rg, trg, head, link) {
-		if (rg->to <= f)
+		/*
+		 * Skip regions before the range to be deleted.  file_region
+		 * ranges are normally of the form [from, to).  However, there
+		 * may be a "placeholder" entry in the map which is of the form
+		 * (from, to) with from == to.  Check for placeholder entries
+		 * at the beginning of the range to be deleted.
+		 */
+		if (rg->to <= f && (rg->to != rg->from || rg->to != f))
 			continue;
+
 		if (rg->from >= t)
 			break;
 
-- 
2.4.3

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