linux-ext4 - Re: [PATCH v2 2/2] jbd2: do not clear the BH_Mapped flag when forgetting a metadata buffer

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Date:   Wed, 12 Feb 2020 11:48:55 +0100
From:   Jan Kara <jack@...e.cz>
To:     "zhangyi (F)" <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, jack@...e.cz, tytso@....edu,
        luoshijie1@...wei.com, zhangxiaoxu5@...wei.com
Subject: Re: [PATCH v2 2/2] jbd2: do not clear the BH_Mapped flag when
 forgetting a metadata buffer

On Tue 11-02-20 21:55:00, 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@...wei.com>
> ---
>  fs/jbd2/commit.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index ecc2ea5f1b59..8f6f4ddd8b78 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -985,12 +985,24 @@ 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;
> +			/*
> +			 * We can (and need to) unmap buffer only for normal
> +			 * mappings. Block device buffers need to stay mapped
> +			 * all the time. We need to be careful about the check
> +			 * because the data page mapping can get cleared under
> +			 * our hands.
> +			 */
> +			mapping = READ_ONCE(bh->b_page->mapping);
> +			if (!(mapping && sb_is_blkdev_sb(mapping->host->i_sb))) {

As I wrote in my other email, we don't have to be that aggressive and this
condition could actually be (mapping && !sb_is_blkdev_sb()) but I guess it
doesn't really matter. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

									Honza

> +				clear_buffer_mapped(bh);
> +				clear_buffer_new(bh);
> +				clear_buffer_req(bh);
> +				bh->b_bdev = NULL;
> +			}
>  		}
>  
>  		if (buffer_jbddirty(bh)) {
> -- 
> 2.17.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR