Re: [RFC] some page can't be migrated

[Andrew Morton]

> On Fri, 25 Jan 2008 14:03:25 +0800 Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
> 
> -	if (!page->mapping)
> +	if (!page->mapping) {
> +		if (!PageAnon(page) && PagePrivate(page))
> +			try_to_release_page(page, GFP_KERNEL);
>  		goto rcu_unlock;
> +	}

We call something(GFP_KERNEL) under rcu_read_lock()?  I've lost track of
the myriad flavours of rcu which we purport to support, but I don't think
they'll all like us blocking under rcu_read_lock().

We _won't_ block, because try_to_release_page() will see the NULL ->mapping
and will call the non-blocking try_to_free_buffers().  But still, it looks
bad, and will cause problems if someone decides to add a might_sleep_if()
to try_to_release_page().

So...  I'd suggest that it would be better to add an apologetic comment and
call direct into try_to_free_buffers().

> -	 * This is a corner case handling.
> -	 * When a new swap-cache is read into, it is linked to LRU
> +	 * There are corner cases handling.
> +	 * 1. When a new swap-cache is read into, it is linked to LRU

hm, that didn't improve the rather flakey grammar in there.  Oh well.

> Acked-by: Nick Piggin <npiggin@xxxxxxx>
> Acked-by: Christoph Lameter <clameter@xxxxxxx>

hm.



Please review and test....

--- a/mm/migrate.c~page-migraton-handle-orphaned-pages-fix
+++ a/mm/migrate.c
@@ -650,20 +650,28 @@ static int unmap_and_move(new_page_t get
 	}
 
 	/*
-	 * There are corner cases handling.
-	 * 1. When a new swap-cache is read into, it is linked to LRU
-	 * and treated as swapcache but has no rmap yet.
-	 * Calling try_to_unmap() against a page->mapping==NULL page is
-	 * BUG. So handle it here.
-	 * 2. Orphaned page (see truncate_complete_page) might have
-	 * fs-private metadata, the page is truncated. The page can be picked
-	 * up due to memory offlineing. Everywhere else except page reclaim,
-	 * the page is invisible to the vm, so the page can't be migrated. Try
-	 * to free metadata, so the page can be freed.
+	 * Corner case handling:
+	 * 1. When a new swap-cache page is read into, it is added to the LRU
+	 * and treated as swapcache but it has no rmap yet.
+	 * Calling try_to_unmap() against a page->mapping==NULL page will
+	 * trigger a BUG.  So handle it here.
+	 * 2. An orphaned page (see truncate_complete_page) might have
+	 * fs-private metadata. The page can be picked up due to memory
+	 * offlining.  Everywhere else except page reclaim, the page is
+	 * invisible to the vm, so the page can not be migrated.  So try to
+	 * free the metadata, so the page can be freed.
 	 */
 	if (!page->mapping) {
-		if (!PageAnon(page) && PagePrivate(page))
-			try_to_release_page(page, GFP_KERNEL);
+		if (!PageAnon(page) && PagePrivate(page)) {
+			/*
+			 * Go direct to try_to_free_buffers() here because
+			 * a) that's what try_to_release_page() would do anyway
+			 * b) we may be under rcu_read_lock() here, so we can't
+			 *    use GFP_KERNEL which is what try_to_release_page()
+			 *    needs to be effective.
+			 */
+			try_to_release_buffers(page);
+		}
 		goto rcu_unlock;
 	}
 

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