linux-kernel - Re: [PATCH v11 1/7] mm: adjust address_space

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Date:	Wed, 7 Nov 2012 19:39:30 -0200
From:	Rafael Aquini <aquini@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v11 1/7] mm: adjust
 address_space_operations.migratepage() return code

On Wed, Nov 07, 2012 at 11:56:10AM -0800, Andrew Morton wrote:
> On Wed,  7 Nov 2012 01:05:48 -0200
> Rafael Aquini <aquini@...hat.com> wrote:
> 
> > This patch introduces MIGRATEPAGE_SUCCESS as the default return code
> > for address_space_operations.migratepage() method and documents the
> > expected return code for the same method in failure cases.
> 
> I hit a large number of rejects applying this against linux-next.  Due
> to the increasingly irritating sched/numa code in there.
> 
> I attempted to fix it up and also converted some (but not all) of the
> implicit tests of `rc' against zero.
> 
> Please check the result very carefully - more changes will be needed.
> 
> All those
> 
> -	if (rc)
> +	if (rc != MIGRATEPAGE_SUCCESS)
> 
> changes are a pain.  Perhaps we shouldn't bother.

Thanks for doing that.

This hunk at migrate_pages(), however, is not necessary:

@@ -1001,7 +1001,7 @@ out:
        if (!swapwrite)
               	current->flags &= ~PF_SWAPWRITE;

-	if (rc)
+       if (rc != MIGRATEPAGE_SUCCESS)
                return rc;

Here, migrate_pages() is not testing rc for the migration success, but it's just trying to
devise the flow if it has to return -ENOMEM, actually.

I guess, a change to make that snippet more clear could be:

diff --git a/mm/migrate.c b/mm/migrate.c
index 77ed2d7..6562aee 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -987,7 +987,7 @@ int migrate_pages(struct list_head *from,
                        case -EAGAIN:
                                retry++;
                                break;
-                       case 0:
+                       case MIGRATEPAGE_SUCCESS:
                                break;
                        default:
                                /* Permanent failure */
@@ -996,15 +996,12 @@ int migrate_pages(struct list_head *from,
                        }
                }
        }
-       rc = 0;
+       rc = nr_failed + retry;
 out:
        if (!swapwrite)
                current->flags &= ~PF_SWAPWRITE;
 
-       if (rc)
-               return rc;
-
-       return nr_failed + retry;
+       return rc;
 }


I can rebase this patch and resubmit if you prefer
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