[PATCH RFC] mm: allow __GFP_NOFAIL allocation up to BLK_MAX_BLOCK_SIZE to support LBS

[libaokun@huaweicloud.com]

From: Baokun Li <libaokun1@huawei.com>

Filesystems use __GFP_NOFAIL to allocate block-sized folios for metadata
reads at critical points, since they cannot afford to go read-only,
shut down, or enter an inconsistent state due to memory pressure.

Currently, attempting to allocate page units greater than order-1 with
the __GFP_NOFAIL flag triggers a WARN_ON() in __alloc_pages_slowpath().
However, filesystems supporting large block sizes (blocksize > PAGE_SIZE)
can easily require allocations larger than order-1.

As Matthew noted, if we have a filesystem with 64KiB sectors, there will
be many clean folios in the page cache that are 64KiB or larger.

Therefore, to avoid the warning when LBS is enabled, we relax this
restriction to allow allocations up to BLK_MAX_BLOCK_SIZE. The current
maximum supported logical block size is 64KiB, meaning the maximum order
handled here is 4.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Link: https://lore.kernel.org/all/aQPX1-XWQjKaMTZB@casper.infradead.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
---
 mm/page_alloc.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fb91c566327c..913b9baa24b4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4663,6 +4663,25 @@ check_retry_cpuset(int cpuset_mems_cookie, struct alloc_context *ac)
 	return false;
 }
 
+/*
+ * We most definitely don't want callers attempting to
+ * allocate greater than order-1 page units with __GFP_NOFAIL.
+ *
+ * However, folio allocations up to BLK_MAX_BLOCK_SIZE with
+ * __GFP_NOFAIL should always be supported.
+ */
+static inline void check_nofail_max_order(unsigned int order)
+{
+	unsigned int max_order = 1;
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	if (PAGE_SIZE << 1 < SZ_64K)
+		max_order = get_order(SZ_64K);
+#endif
+
+	WARN_ON_ONCE(order > max_order);
+}
+
 static inline struct page *
 __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 						struct alloc_context *ac)
@@ -4683,11 +4702,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	int reserve_flags;
 
 	if (unlikely(nofail)) {
-		/*
-		 * We most definitely don't want callers attempting to
-		 * allocate greater than order-1 page units with __GFP_NOFAIL.
-		 */
-		WARN_ON_ONCE(order > 1);
+		check_nofail_max_order(order);
 		/*
 		 * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
 		 * otherwise, we may result in lockup.
-- 
2.46.1