mm: gup: fix potential pgmap refcnt leak in __gup_device_huge()

mainline inclusion
from mainline-v5.15-rc1
commit 6401c4eb
category: bugfix
bugzilla: 180689, https://gitee.com/openeuler/kernel/issues/I53CMX
CVE: NA
backport: openEuler-22.03-LTS

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6401c4eb57f947a49eb144b5b0787cde3318e82e

--------------------------------

When failed to try_grab_page, put_dev_pagemap() is missed.  So pgmap
refcnt will leak in this case.  Also we remove the check for pgmap against
NULL as it's also checked inside the put_dev_pagemap().

[akpm@linux-foundation.org: simplify, cleanup]
[akpm@linux-foundation.org: fix return value]

Link: https://lkml.kernel.org/r/20210807093620.21347-5-linmiaohe@huawei.comSigned-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Fixes: 3faa52c0 ("mm/gup: track FOLL_PIN pages")
Reviewed-by: NJohn Hubbard <jhubbard@nvidia.com>
Reviewed-by: NClaudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 6401c4eb)
Signed-off-by: NYue Zou <zouyue3@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

mm/gup.c

@@ -2319,6 +2319,7 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
 {
 	int nr_start = *nr;
 	struct dev_pagemap *pgmap = NULL;
+	int ret = 1;
 
 	do {
 		struct page *page = pfn_to_page(pfn);
@@ -2326,21 +2327,22 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
 		pgmap = get_dev_pagemap(pfn, pgmap);
 		if (unlikely(!pgmap)) {
 			undo_dev_pagemap(nr, nr_start, flags, pages);
-			return 0;
+			ret = 0;
+			break;
 		}
 		SetPageReferenced(page);
 		pages[*nr] = page;
 		if (unlikely(!try_grab_page(page, flags))) {
 			undo_dev_pagemap(nr, nr_start, flags, pages);
-			return 0;
+			ret = 0;
+			break;
 		}
 		(*nr)++;
 		pfn++;
 	} while (addr += PAGE_SIZE, addr != end);
 
-	if (pgmap)
 	put_dev_pagemap(pgmap);
-	return 1;
+	return ret;
 }
 
 static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,