linux-kernel - Re: [PATCH v2 07/17] fs: new infrastructure for writeback error handling and reporting

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Message-ID: <87inm9uw8b.fsf@notabene.neil.brown.name>
Date:   Thu, 13 Apr 2017 07:55:32 +1000
From:   NeilBrown <neilb@...e.com>
To:     Jeff Layton <jlayton@...hat.com>, linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        tytso@....edu, jack@...e.cz, willy@...radead.org,
        viro@...iv.linux.org.uk
Subject: Re: [PATCH v2 07/17] fs: new infrastructure for writeback error handling and reporting

On Wed, Apr 12 2017, Jeff Layton wrote:


> +void __filemap_set_wb_error(struct address_space *mapping, int err)

I was really hoping that this would be

  void __set_wb_error(wb_err_t *wb_err, int err)

so

Then nfs_context_set_write_error could become

static void nfs_context_set_write_error(struct nfs_open_context *ctx, int error)
{
	__set_wb_error(&ctx->wb_err, error);
}

and filemap_set_sb_error() would be:

static inline void filemap_set_wb_error(struct address_space *mapping, int err)
{
	/* Optimize for the common case of no error */
	if (unlikely(err))
		__set_wb_error(&mapping->f_wb_err, err);
}

Similarly we would have
  wb_err_t sample_wb_error(wb_err_t *wb_err)
  {
   ...
  }

and

wb_err_t filemap_sample_wb_error(struct address_space *mapping)
{
  return sample_wb_error(&mapping->f_wb_err);
}

so nfs_file_fsync_commit() could have
  ret = sample_wb_error(&ctx->wb_err);
in place of
	ret = xchg(&ctx->error, 0);

int filemap_report_wb_error(struct file *file)

would become

int filemap_report_wb_error(struct file *file, wb_err_t *err)

or something.

The address space is just one (obvious) place where the wb error can be
stored.  The filesystem might have a different place with finer
granularity (nfs already does).


> +wb_err_t filemap_sample_wb_error(struct address_space *mapping)
> +{
> +	wb_err_t old = READ_ONCE(mapping->wb_err);
> +	wb_err_t new = old;
> +
> +	/*
> +	 * For the common case of no errors ever having been set, we can skip
> +	 * marking the SEEN bit. Once an error has been set, the value will
> +	 * never go back to zero.
> +	 */
> +	if (old != 0) {
> +		new |= WB_ERR_SEEN;
> +		if (old != new)
> +			cmpxchg(&mapping->wb_err, old, new);
> +	}
> +	return new;
> +}

I do like how the use of cmpxchg work out here - no looping!

Thanks
NeilBrown

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