[PATCH 6.3.y] mm/hugetlb: revert use of page_cache_next_miss()

[Sidhartha Kumar]

As reported by Ackerley[1], the use of page_cache_next_miss() in
hugetlbfs_fallocate() introduces a bug where a second fallocate() call to
same offset fails with -EEXIST. Revert this change and go back to the
previous method of using get from the page cache and then dropping the
reference on success.

hugetlbfs_pagecache_present() was also refactored to use
page_cache_next_miss(), revert the usage there as well.

User visible impacts include hugetlb fallocate incorrectly returning
EEXIST if pages are already present in the file. In addition, hugetlb
pages will not be included in core dumps if they need to be brought in via
GUP. userfaultfd UFFDIO_COPY also uses this code and will not notice pages
already present in the cache. It may try to allocate a new page and
potentially return ENOMEM as opposed to EEXIST.

Fixes: d0ce0e47b323 ("mm/hugetlb: convert hugetlb fault paths to use alloc_hugetlb_folio()")
Cc: <stable@xxxxxxxxxxxxxxx> #v6.3
Reported-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

[1] https://lore.kernel.org/linux-mm/cover.1683069252.git.ackerleytng@xxxxxxxxxx/
---

This revert is the safest way to fix 6.3. The upstream fix will either
fix page_cache_next_miss() itself or use Ackerley's patch to introduce a
new function to check if a page is present in the page cache. Both
directions are currently under review so we can use this safe and simple
fix for 6.3

 fs/hugetlbfs/inode.c |  8 +++-----
 mm/hugetlb.c         | 11 +++++------
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 9062da6da5675..586767afb4cdb 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -821,7 +821,6 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
 		 */
 		struct folio *folio;
 		unsigned long addr;
-		bool present;
 
 		cond_resched();
 
@@ -845,10 +844,9 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
 		mutex_lock(&hugetlb_fault_mutex_table[hash]);
 
 		/* See if already present in mapping to avoid alloc/free */
-		rcu_read_lock();
-		present = page_cache_next_miss(mapping, index, 1) != index;
-		rcu_read_unlock();
-		if (present) {
+		folio = filemap_get_folio(mapping, idx);
+		if (folio) {
+			folio_put(folio);
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 			hugetlb_drop_vma_policy(&pseudo_vma);
 			continue;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 245038a9fe4ea..29ab27d2a3ef5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5666,13 +5666,12 @@ static bool hugetlbfs_pagecache_present(struct hstate *h,
 {
 	struct address_space *mapping = vma->vm_file->f_mapping;
 	pgoff_t idx = vma_hugecache_offset(h, vma, address);
-	bool present;
-
-	rcu_read_lock();
-	present = page_cache_next_miss(mapping, idx, 1) != idx;
-	rcu_read_unlock();
+	struct folio *folio;
 
-	return present;
+	folio = filemap_get_folio(mapping, idx);
+	if (folio)
+		folio_put(folio);
+	return folio != NULL;
 }
 
 int hugetlb_add_to_page_cache(struct folio *folio, struct address_space *mapping,
-- 
2.40.1