[PATCH 6/9] huge tmpfs: SGP_NOALLOC to stop collapse_file() on race

[Hugh Dickins <hughd@google.com>]

khugepaged's collapse_file() currently uses SGP_NOHUGE to tell
shmem_getpage() not to try allocating a huge page, in the very unlikely
event that a racing hole-punch removes the swapped or fallocated page as
soon as i_pages lock is dropped.

We want to consolidate shmem's huge decisions, removing SGP_HUGE and
SGP_NOHUGE; but cannot quite persuade ourselves that it's okay to regress
the protection in this case - Yang Shi points out that the huge page
would remain indefinitely, charged to root instead of the intended memcg.

collapse_file() should not even allocate a small page in this case: why
proceed if someone is punching a hole?  SGP_READ is almost the right flag
here, except that it optimizes away from a fallocated page, with NULL to
tell caller to fill with zeroes (like a hole); whereas collapse_file()'s
sequence relies on using a cache page.  Add SGP_NOALLOC just for this.

There are too many consecutive "if (page"s there in shmem_getpage_gfp():
group it better; and fix the outdated "bring it back from swap" comment.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 include/linux/shmem_fs.h |  1 +
 mm/khugepaged.c          |  2 +-
 mm/shmem.c               | 29 +++++++++++++++++------------
 3 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 9b7f7ac52351..7d97b15a2f7a 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -94,6 +94,7 @@ extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
 /* Flag allocation requirements to shmem_getpage */
 enum sgp_type {
 	SGP_READ,	/* don't exceed i_size, don't allocate page */
+	SGP_NOALLOC,	/* similar, but fail on hole or use fallocated page */
 	SGP_CACHE,	/* don't exceed i_size, may allocate page */
 	SGP_NOHUGE,	/* like SGP_CACHE, but no huge pages */
 	SGP_HUGE,	/* like SGP_CACHE, huge pages preferred */
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b0412be08fa2..045cc579f724 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1721,7 +1721,7 @@ static void collapse_file(struct mm_struct *mm,
 				xas_unlock_irq(&xas);
 				/* swap in or instantiate fallocated page */
 				if (shmem_getpage(mapping->host, index, &page,
-						  SGP_NOHUGE)) {
+						  SGP_NOALLOC)) {
 					result = SCAN_FAIL;
 					goto xa_unlocked;
 				}
diff --git a/mm/shmem.c b/mm/shmem.c
index 740d48ef1eb5..226ac3a911e9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1871,26 +1871,31 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 		return error;
 	}
 
-	if (page)
+	if (page) {
 		hindex = page->index;
-	if (page && sgp == SGP_WRITE)
-		mark_page_accessed(page);
-
-	/* fallocated page? */
-	if (page && !PageUptodate(page)) {
+		if (sgp == SGP_WRITE)
+			mark_page_accessed(page);
+		if (PageUptodate(page))
+			goto out;
+		/* fallocated page */
 		if (sgp != SGP_READ)
 			goto clear;
 		unlock_page(page);
 		put_page(page);
-		page = NULL;
-		hindex = index;
 	}
-	if (page || sgp == SGP_READ)
-		goto out;
 
 	/*
-	 * Fast cache lookup did not find it:
-	 * bring it back from swap or allocate.
+	 * SGP_READ: succeed on hole, with NULL page, letting caller zero.
+	 * SGP_NOALLOC: fail on hole, with NULL page, letting caller fail.
+	 */
+	*pagep = NULL;
+	if (sgp == SGP_READ)
+		return 0;
+	if (sgp == SGP_NOALLOC)
+		return -ENOENT;
+
+	/*
+	 * Fast cache lookup and swap lookup did not find it: allocate.
 	 */
 
 	if (vma && userfaultfd_missing(vma)) {
-- 
2.26.2