shmem: move reclaim check early on writepages()

i915_gem requires huge folios to be split when swapping.  However we have
check for usage of writepages() to ensure it used only for swap purposes
later.  Avoid the splits if we're not being called for reclaim, even if
they should in theory not happen.

This makes the conditions easier to follow on shem_writepage().

Link: https://lkml.kernel.org/r/20230309230545.2930737-4-mcgrof@kernel.orgSigned-off-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Tested-by: Xin Hao <xhao@linux.alibaba.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Adam Manzanares <a.manzanares@samsung.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shmem.c

@@ -1337,6 +1337,16 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	swp_entry_t swap;
 	pgoff_t index;
 
+	/*
+	 * Our capabilities prevent regular writeback or sync from ever calling
+	 * shmem_writepage; but a stacking filesystem might use ->writepage of
+	 * its underlying filesystem, in which case tmpfs should write out to
+	 * swap only in response to memory pressure, and not for the writeback
+	 * threads or sync.
+	 */
+	if (WARN_ON_ONCE(!wbc->for_reclaim))
+		goto redirty;
+
 	/*
 	 * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or
 	 * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
@@ -1357,18 +1367,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	if (!total_swap_pages)
 		goto redirty;
 
-	/*
-	 * Our capabilities prevent regular writeback or sync from ever calling
-	 * shmem_writepage; but a stacking filesystem might use ->writepage of
-	 * its underlying filesystem, in which case tmpfs should write out to
-	 * swap only in response to memory pressure, and not for the writeback
-	 * threads or sync.
-	 */
-	if (!wbc->for_reclaim) {
-		WARN_ON_ONCE(1);	/* Still happens? Tell us about it! */
-		goto redirty;
-	}
-
 	/*
 	 * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC
 	 * value into swapfile.c, the only way we can correctly account for a