Avoid small buffer copying on write

It is wrong for arc_write_ready() to use zfs_abd_scatter_enabled to
decide whether to reallocate/copy the buffer, because the answer is
OS-specific and depends on the buffer size.  Instead of that use
abd_size_alloc_linear(), moved into public header.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Brian Atkinson <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes #12425

Author: amotin

include/sys/abd.h

@@ -91,6 +91,7 @@ abd_t *abd_alloc_linear(size_t, boolean_t);
 abd_t *abd_alloc_gang(void);
 abd_t *abd_alloc_for_io(size_t, boolean_t);
 abd_t *abd_alloc_sametype(abd_t *, size_t);
+boolean_t abd_size_alloc_linear(size_t);
 void abd_gang_add(abd_t *, abd_t *, boolean_t);
 void abd_free(abd_t *);
 abd_t *abd_get_offset(abd_t *, size_t);

include/sys/abd_impl.h

@@ -68,7 +68,6 @@ abd_t *abd_get_offset_scatter(abd_t *, abd_t *, size_t, size_t);
 void abd_free_struct_impl(abd_t *);
 void abd_alloc_chunks(abd_t *, size_t);
 void abd_free_chunks(abd_t *);
-boolean_t abd_size_alloc_linear(size_t);
 void abd_update_scatter_stats(abd_t *, abd_stats_op_t);
 void abd_update_linear_stats(abd_t *, abd_stats_op_t);
 void abd_verify_scatter(abd_t *);

module/os/freebsd/zfs/abd_os.c

@@ -131,7 +131,7 @@ abd_scatter_chunkcnt(abd_t *abd)
 boolean_t
 abd_size_alloc_linear(size_t size)
 {
-	return (size < zfs_abd_scatter_min_size ? B_TRUE : B_FALSE);
+	return (!zfs_abd_scatter_enabled || size < zfs_abd_scatter_min_size);
 }
 
 void

module/os/linux/zfs/abd_os.c

@@ -638,7 +638,7 @@ abd_alloc_zero_scatter(void)
 boolean_t
 abd_size_alloc_linear(size_t size)
 {
-	return (size < zfs_abd_scatter_min_size ? B_TRUE : B_FALSE);
+	return (!zfs_abd_scatter_enabled || size < zfs_abd_scatter_min_size);
 }
 
 void

module/zfs/abd.c

@@ -181,7 +181,7 @@ abd_free_struct(abd_t *abd)
 abd_t *
 abd_alloc(size_t size, boolean_t is_metadata)
 {
-	if (!zfs_abd_scatter_enabled || abd_size_alloc_linear(size))
+	if (abd_size_alloc_linear(size))
 		return (abd_alloc_linear(size, is_metadata));
 
 	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);

module/zfs/arc.c

@@ -6917,7 +6917,8 @@ arc_write_ready(zio_t *zio)
 		arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT | ARC_HDR_ALLOC_RDATA |
 		    ARC_HDR_USE_RESERVE);
 		abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
-	} else if (zfs_abd_scatter_enabled || !arc_can_share(hdr, buf)) {
+	} else if (!abd_size_alloc_linear(arc_buf_size(buf)) ||
+	    !arc_can_share(hdr, buf)) {
 		/*
 		 * Ideally, we would always copy the io_abd into b_pabd, but the
 		 * user may have disabled compressed ARC, thus we must check the