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

[Geert Uytterhoeven]

Hi Qu,

On Mon, Feb 20, 2023 at 12:50 PM Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote:
> On 2023/2/20 16:53, Geert Uytterhoeven wrote:
> > 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/

So the older version was "v2", and the latest version had no
version indicator, nor changelog, thus assuming v1?
No surprise people end up applying the wrong version...

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