Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear VMAs

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 18 Dec 2013 17:23:03 +0800 Wanpeng Li <liwanp@linux.vnet.ibm.com> wrote:

> >>diff --git a/mm/rmap.c b/mm/rmap.c
> >>index 55c8b8d..1e24813 100644
> >>--- a/mm/rmap.c
> >>+++ b/mm/rmap.c
> >>@@ -1347,6 +1347,7 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
> >>  	unsigned long end;
> >>  	int ret = SWAP_AGAIN;
> >>  	int locked_vma = 0;
> >>+	int we_locked = 0;
> >>
> >>  	address = (vma->vm_start + cursor) & CLUSTER_MASK;
> >>  	end = address + CLUSTER_SIZE;
> >>@@ -1385,9 +1386,15 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
> >>  		BUG_ON(!page || PageAnon(page));
> >>
> >>  		if (locked_vma) {
> >>-			mlock_vma_page(page);   /* no-op if already mlocked */
> >>-			if (page == check_page)
> >>+			if (page != check_page) {
> >>+				we_locked = trylock_page(page);
> >
> >If it's not us who has the page already locked, but somebody else, he
> >might unlock it at this point and then the BUG_ON in mlock_vma_page()
> >will trigger again.

yes, this patch is pretty weak.

> Any better idea is appreciated. ;-)

Remove the BUG_ON() from mlock_vma_page()?  Why was it added?
isolate_lru_page() and putback_lru_page() and *might* require
the page be locked, but I don't immediately see issues?