[RFC PATCH 4/4] mm/shmem: make SGP_NOALLOC succeed on hole like SGP_READ

[Chi Zhiling]

From: Chi Zhiling <chizhiling@xxxxxxxxxx>

Change SGP_NOALLOC to return 0 with NULL folio on hole, matching
SGP_READ/SGP_GET behavior. This simplifies the sgp_type handling by
unifying hole semantics across these three types.

Previously, SGP_NOALLOC returned -ENOENT on hole, while SGP_READ/SGP_GET
returned 0. This inconsistency required special handling in callers like
khugepaged and userfaultfd.

After this change:
- khugepaged: behavior unchanged (checks both error and NULL folio)
- userfaultfd: behavior unchanged (both -ENOENT and NULL are converted
  to -EFAULT before returning to userspace)

Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
---
 include/linux/shmem_fs.h | 2 +-
 mm/khugepaged.c          | 2 +-
 mm/shmem.c               | 9 +++------
 3 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 24698faea5a4..6f7f8b9e2a10 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -166,7 +166,7 @@ extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
 enum sgp_type {
 	SGP_GET,	/* don't exceed i_size, don't allocate page, don't lock */
 	SGP_READ,	/* don't exceed i_size, don't allocate page, lock folio */
-	SGP_NOALLOC,	/* similar, but fail on hole or use fallocated page */
+	SGP_NOALLOC,	/* like SGP_READ, but accept fallocated page */
 	SGP_CACHE,	/* don't exceed i_size, may allocate page */
 	SGP_WRITE,	/* may exceed i_size, may allocate !Uptodate page */
 	SGP_FALLOC,	/* like SGP_WRITE, but make existing page Uptodate */
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b8452dbdb043..3309d1c094df 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1950,7 +1950,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 				xas_unlock_irq(&xas);
 				/* swap in or instantiate fallocated page */
 				if (shmem_get_folio(mapping->host, index, 0,
-						&folio, SGP_NOALLOC)) {
+						&folio, SGP_NOALLOC) || !folio) {
 					result = SCAN_FAIL;
 					goto xa_unlocked;
 				}
diff --git a/mm/shmem.c b/mm/shmem.c
index 4bc4e463ca97..41f5e251f7ed 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2531,14 +2531,11 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	}
 
 	/*
-	 * SGP_READ/SGP_GET: succeed on hole, with NULL folio, letting caller zero.
-	 * SGP_NOALLOC: fail on hole, with NULL folio, letting caller fail.
+	 * SGP_READ/SGP_GET/SGP_NOALLOC: succeed on hole, with NULL folio.
 	 */
 	*foliop = NULL;
-	if (sgp == SGP_READ || sgp == SGP_GET)
+	if (sgp <= SGP_NOALLOC)
 		return 0;
-	if (sgp == SGP_NOALLOC)
-		return -ENOENT;
 
 	/*
 	 * Fast cache lookup and swap lookup did not find it: allocate.
@@ -2666,7 +2663,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
  * When no folio is found, the behavior depends on @sgp:
  *  - for SGP_GET, *@foliop is %NULL and 0 is returned
  *  - for SGP_READ, *@foliop is %NULL and 0 is returned
- *  - for SGP_NOALLOC, *@foliop is %NULL and -ENOENT is returned
+ *  - for SGP_NOALLOC, *@foliop is %NULL and 0 is returned
  *  - for all other flags a new folio is allocated, inserted into the
  *    page cache and returned locked in @foliop.
  *
-- 
2.43.0