hugetlb: clean up code checking for fault/truncation races

With the new hugetlb vma lock in place, it can also be used to handle page
fault races with file truncation.  The lock is taken at the beginning of
the code fault path in read mode.  During truncation, it is taken in write
mode for each vma which has the file mapped.  The file's size (i_size) is
modified before taking the vma lock to unmap.

How are races handled?

The page fault code checks i_size early in processing after taking the vma
lock.  If the fault is beyond i_size, the fault is aborted.  If the fault
is not beyond i_size the fault will continue and a new page will be added
to the file.  It could be that truncation code modifies i_size after the
check in fault code.  That is OK, as truncation code will soon remove the
page.  The truncation code will wait until the fault is finished, as it
must obtain the vma lock in write mode.

This patch cleans up/removes late checks in the fault paths that try to
back out pages racing with truncation.  As noted above, we just let the
truncation code remove the pages.

[mike.kravetz@oracle.com: fix reserve_alloc set but not used compiler warning]
  Link: https://lkml.kernel.org/r/Yyj7HsJWfHDoU24U@monkey
Link: https://lkml.kernel.org/r/20220914221810.95771-10-mike.kravetz@oracle.com
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: James Houghton <jthoughton@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Prakash Sangappa <prakash.sangappa@oracle.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 12 insertions, 19 deletions

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 009ae539b9b2..ed57a029eab0 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -568,26 +568,19 @@ static bool remove_inode_single_folio(struct hstate *h, struct inode *inode,
 
 	folio_lock(folio);
 	/*
-	 * After locking page, make sure mapping is the same.
-	 * We could have raced with page fault populate and
-	 * backout code.
+	 * We must remove the folio from page cache before removing
+	 * the region/ reserve map (hugetlb_unreserve_pages).  In
+	 * rare out of memory conditions, removal of the region/reserve
+	 * map could fail.  Correspondingly, the subpool and global
+	 * reserve usage count can need to be adjusted.
 	 */
-	if (folio_mapping(folio) == mapping) {
-		/*
-		 * We must remove the folio from page cache before removing
-		 * the region/ reserve map (hugetlb_unreserve_pages).  In
-		 * rare out of memory conditions, removal of the region/reserve
-		 * map could fail.  Correspondingly, the subpool and global
-		 * reserve usage count can need to be adjusted.
-		 */
-		VM_BUG_ON(HPageRestoreReserve(&folio->page));
-		hugetlb_delete_from_page_cache(&folio->page);
-		ret = true;
-		if (!truncate_op) {
-			if (unlikely(hugetlb_unreserve_pages(inode, index,
-								index + 1, 1)))
-				hugetlb_fix_reserve_counts(inode);
-		}
+	VM_BUG_ON(HPageRestoreReserve(&folio->page));
+	hugetlb_delete_from_page_cache(&folio->page);
+	ret = true;
+	if (!truncate_op) {
+		if (unlikely(hugetlb_unreserve_pages(inode, index,
+							index + 1, 1)))
+			hugetlb_fix_reserve_counts(inode);
 	}
 
 	folio_unlock(folio);