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

[Qu Wenruo]

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