Re: [PATCH v2 4/4] btrfs: replace btrfs_io_context::raid_map[] with a fixed u64 value

From: Qu Wenruo
Date: Mon Feb 20 2023 - 06:50:56 EST




On 2023/2/20 16:53, Geert Uytterhoeven wrote:
    Hi Qu,

On Tue, 7 Feb 2023, Qu Wenruo wrote:
In btrfs_io_context structure, we have a pointer raid_map, which is to
indicate the logical bytenr for each stripe.

But considering we always call sort_parity_stripes(), the result
raid_map[] is always sorted, thus raid_map[0] is always the logical
bytenr of the full stripe.

So why we waste the space and time (for sorting) for raid_map[]?

This patch will replace btrfs_io_context::raid_map with a single u64
number, full_stripe_start, by:

- Replace btrfs_io_context::raid_map with full_stripe_start

- Replace call sites using raid_map[0] to use full_stripe_start

- Replace call sites using raid_map[i] to compare with nr_data_stripes.

The benefits are:

- Less memory wasted on raid_map
 It's sizeof(u64) * num_stripes vs size(u64).
 It's always a save for at least one u64, and the benefit grows larger
 with num_stripes.

- No more weird alloc_btrfs_io_context() behavior
 As there is only one fixed size + one variable length array.

Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>

Thanks for your patch, which is now commit 4a8c6e8a6dc8ae4c ("btrfs:
replace btrfs_io_context::raid_map with a fixed u64 value") in
next-20230220.

noreply@xxxxxxxxxxxxxx reported several build failures when
building for 32-bit platforms:

    ERROR: modpost: "__umoddi3" [fs/btrfs/btrfs.ko] undefined!

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6556,35 +6532,44 @@ int __btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
    }
    bioc->map_type = map->type;

-    for (i = 0; i < num_stripes; i++) {
-        set_io_stripe(&bioc->stripes[i], map, stripe_index, stripe_offset,
-                  stripe_nr);
-        stripe_index++;
-    }
-
-    /* Build raid_map */
+    /*
+     * For RAID56 full map, we need to make sure the stripes[] follows
+     * the rule that data stripes are all ordered, then followed with P
+     * and Q (if we have).
+     *
+     * It's still mostly the same as other profiles, just with extra
+     * rotataion.
+     */
    if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK && need_raid_map &&
        (need_full_stripe(op) || mirror_num > 1)) {
-        u64 tmp;
-        unsigned rot;
-
-        /* Work out the disk rotation on this stripe-set */
-        rot = stripe_nr % num_stripes;
-
-        /* Fill in the logical address of each stripe */
-        tmp = stripe_nr * data_stripes;
-        for (i = 0; i < data_stripes; i++)
-            bioc->raid_map[(i + rot) % num_stripes] =
-                em->start + ((tmp + i) << BTRFS_STRIPE_LEN_SHIFT);
-
-        bioc->raid_map[(i + rot) % map->num_stripes] = RAID5_P_STRIPE;
-        if (map->type & BTRFS_BLOCK_GROUP_RAID6)
-            bioc->raid_map[(i + rot + 1) % num_stripes] =
-                RAID6_Q_STRIPE;
-
-        sort_parity_stripes(bioc, num_stripes);
+        /*
+         * For RAID56 @stripe_nr is already the number of full stripes
+         * before us, which is also the rotation value (needs to modulo
+         * with num_stripes).
+         *
+         * In this case, we just add @stripe_nr with @i, then do the
+         * modulo, to reduce one modulo call.
+         */
+        bioc->full_stripe_logical = em->start +
+            ((stripe_nr * data_stripes) << BTRFS_STRIPE_LEN_SHIFT);
+        for (i = 0; i < num_stripes; i++) {
+            set_io_stripe(&bioc->stripes[i], map,
+                      (i + stripe_nr) % num_stripes,

As stripe_nr is u64, this is a 64-by-32 modulo operation, which
should be implemented using a helper from include/linux/math64.h
instead.

This is an older version, in the latest version, the @stripe_nr variable is also u32, and I tried compiling the latest branch with i686, it doesn't cause any u64 division problems anymore.

You can find the latest branch in either github or from the mailling list:

https://github.com/adam900710/linux/tree/map_block_refactor

https://lore.kernel.org/linux-btrfs/cover.1676611535.git.wqu@xxxxxxxx/

Thanks,
Qu


+                      stripe_offset, stripe_nr);
+        }
+    } else {
+        /*
+         * For all other non-RAID56 profiles, just copy the target
+         * stripe into the bioc.
+         */
+        for (i = 0; i < num_stripes; i++) {
+            set_io_stripe(&bioc->stripes[i], map, stripe_index,
+                      stripe_offset, stripe_nr);
+            stripe_index++;
+        }
    }

+
    if (need_full_stripe(op))
        max_errors = btrfs_chunk_max_errors(map);

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds