linux-ext4 - Re: [PATCH] Ext4: fix ENOSPC when both quota and dioread

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Date:   Fri, 9 Nov 2018 11:20:57 -0800
From:   Liu Bo <obuil.liubo@...il.com>
To:     Liu Bo <bo.liu@...ux.alibaba.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH] Ext4: fix ENOSPC when both quota and dioread_nolock are enabled

ping?

thanks,
liubo

On Wed, Oct 31, 2018 at 6:57 PM Liu Bo <bo.liu@...ux.alibaba.com> wrote:
>
> With dioread_nolock, unwritten extents are converted in IO completion, and
> these extents may be merged with siblings.
>
> ext4_writepages() has reserved a handle to hold 2 credits for inode update
> and extent update and the handle will be used in IO completion as we can't
> start journal after clearing PageWriteback (due to locking rules).
>
> However, if quota is enabled, the following enospc situation could happen,
>
> ext4_convert_unwritten_extents
>   ext4_ext_try_to_merge
>     ext4_ext_try_to_merge_up
>       ext4_journal_extend(handle, 2) # extend 2 credits for
>                                      # bitmap/group_desc
>       ext4_free_blocks()
>         dquot_free_block
>           // use 3 credits for 3 quota types
>           // use 1 credit for inode update
>         # update bitmap block
>         # update group desc block
>         # either of two updates may hit enospc
>         # as ->h_buffer_credits has been consumed out.
>
> In ext4_ext_try_to_merge(), per-inode extent tree may be collasped into
> inode and ext4_free_blocks() will then free the unneeded blocks, but with
> quota being enabled, there aren't enough credits to process updates and we
> end up with -ENOSPC.
>
> As this is a corner case, instead of adding credits in ext4_writepages(),
> this just extends more credits for quota when doing merge-up.
>
> Fixes: ecb94f5fdf4b (ext4: collapse a single extent tree block into the inode if possible)
> CC: stable@...r.kernel.org # v3.6-rc1+
> Signed-off-by: Liu Bo <bo.liu@...ux.alibaba.com>
> ---
>  fs/ext4/extents.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 240b6dea5441..ff2498355e3c 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -1812,6 +1812,7 @@ static void ext4_ext_try_to_merge_up(handle_t *handle,
>         size_t s;
>         unsigned max_root = ext4_ext_space_root(inode, 0);
>         ext4_fsblk_t blk;
> +       int credits = 2;
>
>         if ((path[0].p_depth != 1) ||
>             (le16_to_cpu(path[0].p_hdr->eh_entries) != 1) ||
> @@ -1820,10 +1821,12 @@ static void ext4_ext_try_to_merge_up(handle_t *handle,
>
>         /*
>          * We need to modify the block allocation bitmap and the block
> -        * group descriptor to release the extent tree block.  If we
> +        * group descriptor to release the extent tree block.  If
> +        * quota is enabled, updates on quota are also needed.  If we
>          * can't get the journal credits, give up.
>          */
> -       if (ext4_journal_extend(handle, 2))
> +       credits += EXT4_MAXQUOTAS_TRANS_BLOCKS(inode->i_sb);
> +       if (ext4_journal_extend(handle, credits))
>                 return;
>
>         /*
> --
> 1.8.3.1
>