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Date: Mon, 20 Feb 2023 19:50:37 +0800
From: Qu Wenruo <quwenruo.btrfs@....com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>, Qu Wenruo <wqu@...e.com>
Cc: David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
linux-next@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] btrfs: replace btrfs_io_context::raid_map[] with a
fixed u64 value
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@...e.com>
>
> 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@...erman.id.au 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@suse.com/
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@...ux-m68k.org
>
> 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
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