[PATCH next 1/3] slub defrag: unpin writeback pages

[Hugh Dickins <hugh@veritas.com>]

A repetitive swapping load on powerpc G5 went progressively slower after
nine hours: Inactive(file) was rising, as if inactive file pages pinned.
Yes, slub defrag's kick_buffers() was forgetting to put_page() whenever
it met a page already under writeback.

That PageWriteback test should be made while PageLocked in trigger_write(),
just as it is in try_to_free_buffers() - if there are complex reasons why
that's not actually necessary, I'd rather not have to think through them.
A preliminary check before taking the lock?  No, it's not that important.

And trigger_write() must remember to unlock_page() in each of the cases
where it doesn't reach the writepage().

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
Andrew, I mentioned at KS that I'd just completed a slow -mm bisection,
which had mysteriously converged on one of your patches: that was your
vm-dont-run-touch_buffer-during-buffercache-lookups.patch
In fact that turned out just to speed up the rate at which writebacks
were pinning these pages: I've nothing against (nor for!) your patch.

I'm wondering whether kick_buffers() ought to check PageLRU: I've no
evidence it's needed, but coming to writepage() or try_to_free_buffers()
from this direction (by virtue of having a buffer_head in a partial slab
page) is new, and I worry it might cause some ordering problem; whereas
if the page is PageLRU, then it is already fair game for such reclaim.

 fs/buffer.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- 2.6.27-rc7-mmotm/fs/buffer.c	2008-09-26 13:18:50.000000000 +0100
+++ linux/fs/buffer.c	2008-10-03 19:43:44.000000000 +0100
@@ -3354,13 +3354,16 @@ static void trigger_write(struct page *p
 		.for_reclaim = 0
 	};
 
+	if (PageWriteback(page))
+		goto unlock;
+
 	if (!mapping->a_ops->writepage)
 		/* No write method for the address space */
-		return;
+		goto unlock;
 
 	if (!clear_page_dirty_for_io(page))
 		/* Someone else already triggered a write */
-		return;
+		goto unlock;
 
 	rc = mapping->a_ops->writepage(page, &wbc);
 	if (rc < 0)
@@ -3368,7 +3371,7 @@ static void trigger_write(struct page *p
 		return;
 
 	if (rc == AOP_WRITEPAGE_ACTIVATE)
-		unlock_page(page);
+unlock:		unlock_page(page);
 }
 
 /*
@@ -3420,7 +3423,7 @@ static void kick_buffers(struct kmem_cac
 	for (i = 0; i < nr; i++) {
 		page = v[i];
 
-		if (!page || PageWriteback(page))
+		if (!page)
 			continue;
 
 		if (trylock_page(page)) {

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