Re: [PATCH] mm: swap: Mark swap pages writeback before queueing for direct IO

[Mel Gorman <mgorman@suse.de>]

On Wed, Apr 24, 2013 at 12:23:13PM -0700, Andrew Morton wrote:
> >  		} else {
> > +			/*
> > +			 * In the case of swap-over-nfs, this can be a
> > +			 * temporary failure if the system has limited
> > +			 * memory for allocating transmit buffers.
> > +			 * Mark the page dirty and avoid
> > +			 * rotate_reclaimable_page but rate-limit the
> > +			 * messages but do not flag PageError like
> > +			 * the normal direct-to-bio case as it could
> > +			 * be temporary.
> > +			 */
> >  			set_page_dirty(page);
> > +			ClearPageReclaim(page);
> > +			if (printk_ratelimit()) {
> > +				pr_err("Write-error on dio swapfile (%Lu)\n",
> > +					(unsigned long long)page_file_offset(page));
> > +			}
> >  		}
> > +		end_page_writeback(page);
> 
> A pox upon printk_ratelimit()!  Both its code comment and the
> checkpatch warning explain why.
> 

Ok. There were few sensible options around dealing with the write
errors. swap_writepage() could go to sleep on a waitqueue but it's
putting IO rate limiting where it doesn't belong. Retrying silently
forever could be difficult to debug if the error really is permanent.

> --- a/mm/page_io.c~mm-swap-mark-swap-pages-writeback-before-queueing-for-direct-io-fix
> +++ a/mm/page_io.c
> @@ -244,10 +244,8 @@ int __swap_writepage(struct page *page,
>  			 */
>  			set_page_dirty(page);
>  			ClearPageReclaim(page);
> -			if (printk_ratelimit()) {
> -				pr_err("Write-error on dio swapfile (%Lu)\n",
> -					(unsigned long long)page_file_offset(page));
> -			}
> +			pr_err_ratelimited("Write error on dio swapfile (%Lu)\n",
> +				(unsigned long long)page_file_offset(page));
>  		}
>  		end_page_writeback(page);
>  		return ret;
> 
> Do we need to cast the loff_t?  afaict all architectures use long long.
> I didn't get a warning from sparc64 with the cast removed, and sparc64
> is the one which likes to use different underlying types.
> 
> I think I'll remove it and wait for Fengguang's nastygram.
> 

Sounds reasonable. I'll get cc'd on the same mails.

-- 
Mel Gorman
SUSE Labs

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