xfs: refactor backing memory allocations for buffers

Lift handling of shmem and slab backed buffers into xfs_buf_alloc_pages
and rename the result to xfs_buf_alloc_backing_mem.  This shares more
code and ensures uncached buffers can also use slab, which slightly
reduces the memory usage of growfs on 512 byte sector size file systems,
but more importantly means the allocation invariants are the same for
cached and uncached buffers.  Document these new invariants with a big
fat comment mostly stolen from a patch by Dave Chinner.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Christoph Hellwig

fs/xfs/xfs_buf.c

@@ -271,19 +271,49 @@ xfs_buf_alloc_kmem(
 	return 0;
 }
 
+/*
+ * Allocate backing memory for a buffer.
+ *
+ * For tmpfs-backed buffers used by in-memory btrees this directly maps the
+ * tmpfs page cache folios.
+ *
+ * For real file system buffers there are two different kinds backing memory:
+ *
+ * The first type backs the buffer by a kmalloc allocation.  This is done for
+ * less than PAGE_SIZE allocations to avoid wasting memory.
+ *
+ * The second type of buffer is the multi-page buffer. These are always made
+ * up of single pages so that they can be fed to vmap_ram() to return a
+ * contiguous memory region we can access the data through, or mark it as
+ * XBF_UNMAPPED and access the data directly through individual page_address()
+ * calls.
+ */
 static int
-xfs_buf_alloc_pages(
+xfs_buf_alloc_backing_mem(
 	struct xfs_buf	*bp,
 	xfs_buf_flags_t	flags)
 {
+	size_t		size = BBTOB(bp->b_length);
 	gfp_t		gfp_mask = GFP_KERNEL | __GFP_NOLOCKDEP | __GFP_NOWARN;
 	long		filled = 0;
 
+	if (xfs_buftarg_is_mem(bp->b_target))
+		return xmbuf_map_page(bp);
+
+	/*
+	 * For buffers that fit entirely within a single page, first attempt to
+	 * allocate the memory from the heap to minimise memory usage.  If we
+	 * can't get heap memory for these small buffers, we fall back to using
+	 * the page allocator.
+	 */
+	if (size < PAGE_SIZE && xfs_buf_alloc_kmem(new_bp, flags) == 0)
+		return 0;
+
 	if (flags & XBF_READ_AHEAD)
 		gfp_mask |= __GFP_NORETRY;
 
 	/* Make sure that we have a page list */
-	bp->b_page_count = DIV_ROUND_UP(BBTOB(bp->b_length), PAGE_SIZE);
+	bp->b_page_count = DIV_ROUND_UP(size, PAGE_SIZE);
 	if (bp->b_page_count <= XB_PAGES) {
 		bp->b_pages = bp->b_page_array;
 	} else {
@@ -564,18 +594,7 @@ xfs_buf_find_insert(
 	if (error)
 		goto out_drop_pag;
 
-	if (xfs_buftarg_is_mem(new_bp->b_target)) {
-		error = xmbuf_map_page(new_bp);
-	} else if (BBTOB(new_bp->b_length) >= PAGE_SIZE ||
-		   xfs_buf_alloc_kmem(new_bp, flags) < 0) {
-		/*
-		 * For buffers that fit entirely within a single page, first
-		 * attempt to allocate the memory from the heap to minimise
-		 * memory usage. If we can't get heap memory for these small
-		 * buffers, we fall back to using the page allocator.
-		 */
-		error = xfs_buf_alloc_pages(new_bp, flags);
-	}
+	error = xfs_buf_alloc_backing_mem(new_bp, flags);
 	if (error)
 		goto out_free_buf;
 
@@ -939,14 +958,12 @@ xfs_buf_get_uncached(
 	if (error)
 		return error;
 
-	if (xfs_buftarg_is_mem(bp->b_target))
-		error = xmbuf_map_page(bp);
-	else
-		error = xfs_buf_alloc_pages(bp, flags);
+	error = xfs_buf_alloc_backing_mem(bp, flags);
 	if (error)
 		goto fail_free_buf;
 
-	error = _xfs_buf_map_pages(bp, 0);
+	if (!bp->b_addr)
+		error = _xfs_buf_map_pages(bp, 0);
 	if (unlikely(error)) {
 		xfs_warn(target->bt_mount,
 			"%s: failed to map pages", __func__);