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

[Ric Mason <ric.masonn@gmail.com>]

Hi Mel,
On 04/25/2013 02:57 AM, Mel Gorman wrote:
> As pointed out by Andrew Morton, the swap-over-NFS writeback is not setting
> PageWriteback before it is queued for direct IO. While swap pages do not

Before commit commit 62c230bc1 (mm: add support for a filesystem to 
activate swap files and use direct_IO for writing swap pages), swap 
pages will write to page cache firstly and then writeback?

> participate in BDI or process dirty accounting and the IO is synchronous,
> the writeback bit is still required and not setting it in this case was
> an oversight.  swapoff depends on the page writeback to synchronoise all
> pending writes on a swap page before it is reused. Swapcache freeing and
> reuse depend on checking the PageWriteback under lock to ensure the page
> is safe to reuse.
>
> Direct IO handlers and the direct IO handler for NFS do not deal with
> PageWriteback as they are synchronous writes. In the case of NFS, it
> schedules pages (or a page in the case of swap) for IO and then waits
> synchronously for IO to complete in nfs_direct_write(). It is recognised
> that this is a slowdown from normal swap handling which is asynchronous
> and uses a completion handler. Shoving PageWriteback handling down into
> direct IO handlers looks like a bad fit to handle the swap case although
> it may have to be dealt with some day if swap is converted to use direct
> IO in general and bmap is finally done away with. At that point it will
> be necessary to refit asynchronous direct IO with completion handlers onto
> the swap subsystem.
>
> As swapcache currently depends on PageWriteback to protect against races,
> this patch sets PageWriteback under the page lock before queueing it for
> direct IO. It is cleared when the direct IO handler returns. IO errors
> are treated similarly to the direct-to-bio case except PageError is not
> set as in the case of swap-over-NFS, it is likely to be a transient error.
>
> It was asked what prevents such a page being reclaimed in parallel.
> With this patch applied, such a page will now be skipped (most of the time)
> or blocked until the writeback completes.  Reclaim checks PageWriteback
> under the page lock before calling try_to_free_swap and the page lock
> should prevent the page being requeued for IO before it is freed.
>
> This and Jerome's related patch should considered for -stable as far
> back as 3.6 when swap-over-NFS was introduced.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>   mm/page_io.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 04ca00d..ec04247 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -214,6 +214,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
>   		kiocb.ki_left = PAGE_SIZE;
>   		kiocb.ki_nbytes = PAGE_SIZE;
>   
> +		set_page_writeback(page);
>   		unlock_page(page);
>   		ret = mapping->a_ops->direct_IO(KERNEL_WRITE,
>   						&kiocb, &iov,
> @@ -223,8 +224,24 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
>   			count_vm_event(PSWPOUT);
>   			ret = 0;
>   		} 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);
>   		return ret;
>   	}
>
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