Re: [PATCH 02/11] filemap: Remove filemap_check_and_keep_errors()

[Jeff Layton <jlayton@redhat.com>]

On Mon, 2023-01-09 at 05:18 +0000, Matthew Wilcox (Oracle) wrote:
> Convert both callers to use the "new" errseq infrastructure.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/filemap.c | 18 ++++++------------
>  1 file changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c4d4ace9cc70..48daedc224d9 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -355,16 +355,6 @@ int filemap_check_errors(struct address_space *mapping)
>  }
>  EXPORT_SYMBOL(filemap_check_errors);
>  
> -static int filemap_check_and_keep_errors(struct address_space *mapping)
> -{
> -	/* Check for outstanding write errors */
> -	if (test_bit(AS_EIO, &mapping->flags))
> -		return -EIO;
> -	if (test_bit(AS_ENOSPC, &mapping->flags))
> -		return -ENOSPC;
> -	return 0;
> -}
> -
>  /**
>   * filemap_fdatawrite_wbc - start writeback on mapping dirty pages in range
>   * @mapping:	address space structure to write
> @@ -567,8 +557,10 @@ EXPORT_SYMBOL(filemap_fdatawait_range);
>  int filemap_fdatawait_range_keep_errors(struct address_space *mapping,
>  		loff_t start_byte, loff_t end_byte)
>  {
> +	errseq_t since = filemap_sample_wb_err(mapping);
> +
>  	__filemap_fdatawait_range(mapping, start_byte, end_byte);
> -	return filemap_check_and_keep_errors(mapping);
> +	return filemap_check_wb_err(mapping, since);
>  }
>  EXPORT_SYMBOL(filemap_fdatawait_range_keep_errors);

I looked at making this sort of change across the board alongside the
original wb_err patches, but I backed off at the time.

With the above patch, this function will no longer report a writeback
error that occurs before the sample. Given that writeback can happen at
any time, that seemed like it might be an undesirable change, and I
didn't follow through.

It is true that the existing flag-based code may miss errors too, if
multiple tasks are test_and_clear'ing the bits, but I think the above is
even more likely to happen, esp. under memory pressure.

To do this right, we probably need to look at these callers and have
them track a long-term errseq_t "since" value before they ever dirty the
pages, and then continually check-and-advance vs. that.

For instance, the main caller of the above function is jbd2. Would it be
reasonable to add in a new errseq_t value to the jnode for tracking
errors?

>  
> @@ -613,8 +605,10 @@ EXPORT_SYMBOL(file_fdatawait_range);
>   */
>  int filemap_fdatawait_keep_errors(struct address_space *mapping)
>  {
> +	errseq_t since = filemap_sample_wb_err(mapping);
> +
>  	__filemap_fdatawait_range(mapping, 0, LLONG_MAX);
> -	return filemap_check_and_keep_errors(mapping);
> +	return filemap_check_wb_err(mapping, since);
>  }
>  EXPORT_SYMBOL(filemap_fdatawait_keep_errors);
>  

-- 
Jeff Layton <jlayton@redhat.com>