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Subject: Re: [PATCH v3 2/2] jbd2: do not clear the BH_Mapped flag when
 forgetting a metadata buffer
To:     "zhangyi (F)" <yi.zhang@huawei.com>, jack@suse.cz, tytso@mit.edu
Cc:     linux-ext4@vger.kernel.org, luoshijie1@huawei.com,
        zhangxiaoxu5@huawei.com
References: <20200213063821.30455-1-yi.zhang@huawei.com>
 <20200213063821.30455-3-yi.zhang@huawei.com>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Tue, 18 Feb 2020 10:48:13 +0530
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On 2/13/20 12:08 PM, zhangyi (F) wrote:
> Commit 904cdbd41d74 ("jbd2: clear dirty flag when revoking a buffer from
> an older transaction") set the BH_Freed flag when forgetting a metadata
> buffer which belongs to the committing transaction, it indicate the
> committing process clear dirty bits when it is done with the buffer. But
> it also clear the BH_Mapped flag at the same time, which may trigger
> below NULL pointer oops when block_size < PAGE_SIZE.
> 
> rmdir 1             kjournald2                 mkdir 2
>                      jbd2_journal_commit_transaction
> 		    commit transaction N
> jbd2_journal_forget
> set_buffer_freed(bh1)
>                      jbd2_journal_commit_transaction
>                       commit transaction N+1
>                       ...
>                       clear_buffer_mapped(bh1)
>                                                 ext4_getblk(bh2 ummapped)
>                                                 ...
>                                                 grow_dev_page
>                                                  init_page_buffers
>                                                   bh1->b_private=NULL
>                                                   bh2->b_private=NULL
>                       jbd2_journal_put_journal_head(jh1)
>                        __journal_remove_journal_head(hb1)
> 		       jh1 is NULL and trigger oops
> 
> *) Dir entry block bh1 and bh2 belongs to one page, and the bh2 has
>     already been unmapped.
> 
> For the metadata buffer we forgetting, we should always keep the mapped
> flag and clear the dirty flags is enough, so this patch pick out the
> these buffers and keep their BH_Mapped flag.
> 
> Fixes: 904cdbd41d74 ("jbd2: clear dirty flag when revoking a buffer from an older transaction")
> Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>

This should be a stable candidate I guess.

-ritesh

> ---
>   fs/jbd2/commit.c | 25 +++++++++++++++++++++----
>   1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 6396fe70085b..27373f5792a4 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -985,12 +985,29 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>   		 * pagesize and it is attached to the last partial page.
>   		 */
>   		if (buffer_freed(bh) && !jh->b_next_transaction) {
> +			struct address_space *mapping;
> +
>   			clear_buffer_freed(bh);
>   			clear_buffer_jbddirty(bh);
> -			clear_buffer_mapped(bh);
> -			clear_buffer_new(bh);
> -			clear_buffer_req(bh);
> -			bh->b_bdev = NULL;
> +
> +			/*
> +			 * Block device buffers need to stay mapped all the
> +			 * time, so it is enough to clear buffer_jbddirty and
> +			 * buffer_freed bits. For the file mapping buffers (i.e.
> +			 * journalled data) we need to unmap buffer and clear
> +			 * more bits. We also need to be careful about the check
> +			 * because the data page mapping can get cleared under
> +			 * out hands, which alse need not to clear more bits
> +			 * because the page and buffers will be freed and can
> +			 * never be reused once we are done with them.
> +			 */
> +			mapping = READ_ONCE(bh->b_page->mapping);
> +			if (mapping && !sb_is_blkdev_sb(mapping->host->i_sb)) {
> +				clear_buffer_mapped(bh);
> +				clear_buffer_new(bh);
> +				clear_buffer_req(bh);
> +				bh->b_bdev = NULL;
> +			}
>   		}
> 
>   		if (buffer_jbddirty(bh)) {
>