From: Carlos Maiolino <cmaiolino@redhat.com>
Subject: Re: ext4_wait_block_bitmap() and ext4_read_block_bitmap_nowait()
 handle bitmap verification differently
Date: Fri, 4 Oct 2013 11:04:08 -0300
Message-ID: <20131004140407.GA1348@orion.maiolino.org>
References: <CAGW2f1HsP1s+Avuiietg1BqqNeEJJ8g1MOgxHnx-8hqK57QG7Q@mail.gmail.com>
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Hi,

looks like that, with this patch, ext4_read_block_bitmap_nowait() might sleep in
wait_on_buffer() called from ext4_wait_block_bitmap(), which is not really the
intention of ext4_read_block_bitmap_nowait().

Am I wrong here or this might really happen?


On Thu, Oct 03, 2013 at 10:45:06PM -0400, jon ernst wrote:
> Hi,
> 
> I found that ext4_wait_block_bitmap() and
> ext4_read_block_bitmap_nowait() handle bitmap verification
> differently.
> wait_block_bitmap() calls ext4_validate_block_bitmap() all the time.
> But  read_block_bitmap_nowait() checks EXT4_BG_BLOCK_UNINIT, if it
> meets, it will skip ext4_validate_block_bitmap()
> 
> In my opinion, they'd better do same thing.
> In that way, we can also return "fail" in ext4_valid_block_bitmap()
> method when we meet FLEX_BG.
> 
> 
> 
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index dc5d572..366807a 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -319,7 +319,7 @@ static ext4_fsblk_t ext4_valid_block_bitmap(struct
> super_block *sb,
>                  * or it has to also read the block group where the bitmaps
>                  * are located to verify they are set.
>                  */
> -               return 0;
> +               return 1;
>         }
>         group_first_block = ext4_group_first_block_no(sb, block_group);
> 
> @@ -472,8 +472,12 @@ int ext4_wait_block_bitmap(struct super_block
> *sb, ext4_group_t block_group,
>                 return 1;
>         }
>         clear_buffer_new(bh);
> -       /* Panic or remount fs read-only if block bitmap is invalid */
> -       ext4_validate_block_bitmap(sb, desc, block_group, bh);
> +
> +       if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
> +        return 0;
> +        }
> +       /* Panic or remount fs read-only if block bitmap is invalid */
> +       ext4_validate_block_bitmap(sb, desc, block_group, bh);
>         /* ...but check for error just in case errors=continue. */
>         return !buffer_verified(bh);
>  }
> --
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-- 
Carlos