Re: [PATCH v3] dm: optimize REQ_PREFLUSH with data when using the linear target

[Bart Van Assche]

On 9/12/25 12:56 PM, Mikulas Patocka wrote:

If the table has only linear targets and there is just one underlying
device, we can optimize REQ_PREFLUSH with data - we don't have to split
it to two bios - a flush and a write. We can pass it to the linear target
directly.

Thanks! Since this patch works fine on my test setup:

Tested-by: Bart Van Assche <bvanass...@acm.org>

@@ -1996,11 +1993,19 @@ static void dm_split_and_process_bio(str
        }
        init_clone_info(&ci, io, map, bio, is_abnormal);

- if (bio->bi_opf & REQ_PREFLUSH) {

+       if (unlikely((bio->bi_opf & REQ_PREFLUSH) != 0)) {
+               if (map->flush_bypasses_map) {

How map->flush_bypasses_map makes submitting REQ_PREFLUSH bios that have
data safe is nontrivial so it's probably a good idea to explain this in
a comment.

+                       struct list_head *devices = dm_table_get_devices(map);
+                       if (devices->next == devices->prev)
+                               goto send_preflush_with_data;

Should it be explained in a comment that devices->next == devices->prev
checks whether there are either no elements or one element in the list
and that we know that the devices list is not empty?

+               }
+               if (bio->bi_iter.bi_size)
+                       io->requeue_flush_with_data = true;
                __send_empty_flush(&ci);
                /* dm_io_complete submits any data associated with flush */
                goto out;
        }
+send_preflush_with_data:

if (static_branch_unlikely(&zoned_enabled) &&

            (bio_op(bio) == REQ_OP_ZONE_RESET_ALL)) {

Isn't it recommended to leave a blank line above goto labels rather than
below?

Thanks,

Bart.