bug report: hugetlbfs: use i_mmap_rwsem for more pmd sharing, synchronization

[Colin Ian King <colin.king@canonical.com>]

Hi,

Static analysis with CoverityScan on linux-next detected a potential
null pointer dereference with the following commit:

>From d8a1051ed4ba55679ef24e838a1942c9c40f0a14 Mon Sep 17 00:00:00 2001
From: Mike Kravetz <mike.kravetz@oracle.com>
Date: Sat, 22 Dec 2018 10:55:57 +1100
Subject: [PATCH] hugetlbfs: use i_mmap_rwsem for more pmd sharing

The earlier check implies that "mapping" may be a null pointer:

var_compare_op: Comparing mapping to null implies that mapping might be
null.

1008        if (!(flags & MF_MUST_KILL) && !PageDirty(hpage) && mapping &&
1009            mapping_cap_writeback_dirty(mapping)) {

..however later "mapper" is dereferenced when it may be potentially null:

1034                /*
1035                 * For hugetlb pages, try_to_unmap could potentially
call
1036                 * huge_pmd_unshare.  Because of this, take semaphore in
1037                 * write mode here and set TTU_RMAP_LOCKED to
indicate we
1038                 * have taken the lock at this higer level.
1039                 */
    CID 1476097 (#1 of 1): Dereference after null check (FORWARD_NULL)

var_deref_model: Passing null pointer mapping to
i_mmap_lock_write, which dereferences it.

1040                i_mmap_lock_write(mapping);
1041                unmap_success = try_to_unmap(hpage,
ttu|TTU_RMAP_LOCKED);
1042                i_mmap_unlock_write(mapping);


Colin