Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Hi

> >> Page reclaim shouldn't be even attempting to reclaim or write back
> >> ramfs pagecache pages - reclaim can't possibly do anything with these
> >> pages!
> >>
> >> Arguably those pages shouldn't be on the LRU at all, but we haven't
> >> done that yet.
> >>
> >> Now, my problem is that I can't 100% be sure that we _ever_ implemented
> >> this properly. ?I _think_ we did, in which case we later broke it. ?If
> >> we've always been (stupidly) trying to pageout these pages then OK, I
> >> guess your patch is a suitable 2.6.29 stopgap.
> >
> > OK, I can't find any code anywhere in which we excluded ramfs pages
> > from consideration by page reclaim. ?How dumb.
> 
> The ramfs  considers it in just CONFIG_UNEVICTABLE_LRU case
> It that case, ramfs_get_inode calls mapping_set_unevictable.
> So,  page reclaim can exclude ramfs pages by page_evictable.
> It's problem .

Currently, CONFIG_UNEVICTABLE_LRU can't use on nommu machine
because nobody of vmscan folk havbe nommu machine.

Yes, it is very stupid reason. _very_ welcome to tester! :)



David, Could you please try following patch if you have NOMMU machine?
it is straightforward porting to nommu.


==
Subject: [PATCH] remove to depend on MMU from CONFIG_UNEVICTABLE_LRU

logically, CONFIG_UNEVICTABLE_LRU doesn't depend on MMU.
but current code does by mistake. fix it.


Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 mm/Kconfig |    1 -
 mm/nommu.c |   24 ++++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

Index: b/mm/Kconfig
===================================================================
--- a/mm/Kconfig	2008-12-28 20:55:23.000000000 +0900
+++ b/mm/Kconfig	2008-12-28 21:24:08.000000000 +0900
@@ -212,7 +212,6 @@ config VIRT_TO_BUS
 config UNEVICTABLE_LRU
 	bool "Add LRU list to track non-evictable pages"
 	default y
-	depends on MMU
 	help
 	  Keeps unevictable pages off of the active and inactive pageout
 	  lists, so kswapd will not waste CPU time or have its balancing
Index: b/mm/nommu.c
===================================================================
--- a/mm/nommu.c	2008-12-25 08:26:37.000000000 +0900
+++ b/mm/nommu.c	2008-12-28 21:29:36.000000000 +0900
@@ -1521,3 +1521,27 @@ int access_process_vm(struct task_struct
 	mmput(mm);
 	return len;
 }
+
+/*
+ *  LRU accounting for clear_page_mlock()
+ */
+void __clear_page_mlock(struct page *page)
+{
+	VM_BUG_ON(!PageLocked(page));
+
+	if (!page->mapping) {	/* truncated ? */
+		return;
+	}
+
+	dec_zone_page_state(page, NR_MLOCK);
+	count_vm_event(UNEVICTABLE_PGCLEARED);
+	if (!isolate_lru_page(page)) {
+		putback_lru_page(page);
+	} else {
+		/*
+		 * We lost the race. the page already moved to evictable list.
+		 */
+		if (PageUnevictable(page))
+			count_vm_event(UNEVICTABLE_PGSTRANDED);
+	}
+}




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