Re: [PATCH 2/6] mm: mlocking in try_to_unmap_one

KOSAKI Motohiro
2009-11-13T08:16:33Z

> On Wed, 11 Nov 2009, KOSAKI Motohiro wrote:
>
> Though it doesn't quite answer your question,
> I'll just reinsert the last paragraph of my description here...
>
> > > try_to_unmap_file()'s TTU_MUNLOCK nonlinear handling was particularly
> > > amusing: once unravelled, it turns out to have been choosing between
> > > two different ways of doing the same nothing. Ah, no, one way was
> > > actually returning SWAP_FAIL when it meant to return SWAP_SUCCESS.
>
> ...
> > > @@ -1081,45 +1053,23 @@ static int try_to_unmap_file(struct page
> ...
> > >
> > > - if (list_empty(&mapping->i_mmap_nonlinear))
> > > + /* We don't bother to try to find the munlocked page in nonlinears */
> > > + if (MLOCK_PAGES && TTU_ACTION(flags) == TTU_MUNLOCK)
> > > goto out;
> >
> > I have dumb question.
> > Does this shortcut exiting code makes any behavior change?
>
> Not dumb. My intention was to make no behaviour change with any of
> this patch; but in checking back before completing the description,
> I suddenly realized that that shortcut intentionally avoids the
>
> if (max_nl_size == 0) { /* all nonlinears locked or reserved ? */
> ret = SWAP_FAIL;
> goto out;
> }
>
> (which doesn't show up in the patch: you'll have to look at rmap.c),
> which used to have the effect of try_to_munlock() returning SWAP_FAIL
> in the case when there were one or more VM_NONLINEAR vmas of the file,
> but none of them (and none of the covering linear vmas) VM_LOCKED.
>
> That should have been a SWAP_SUCCESS case, or with my changes
> another SWAP_AGAIN, either of which would make munlock_vma_page()
> count_vm_event(UNEVICTABLE_PGMUNLOCKED);
> which would be correct; but the SWAP_FAIL meant that count was not
> incremented in this case.

Ah, correct.
Then, we lost the capability unevictability of non linear mapping pages, right.
if so, following additional patch makes more consistent?

>
> Actually, I've double-fixed that, because I also changed
> munlock_vma_page() to increment the count whenever ret != SWAP_MLOCK;
> which seemed more appropriate, but would have been a no-op if
> try_to_munlock() only returned SWAP_SUCCESS or SWAP_AGAIN or SWAP_MLOCK
> as it claimed.
>
> But I wasn't very inclined to boast of fixing that bug, since my testing
> didn't give confidence that those /proc/vmstat unevictable_pgs_*lock*
> counts are being properly maintained anyway - when I locked the same
> pages in two vmas then unlocked them in both, I ended up with mlocked
> bigger than munlocked (with or without my 2/6 patch); which I suspect
> is wrong, but rather off my present course towards KSM swapping...

Ah, vmstat inconsistent is weird. I'll try to debug it later.
Thanks this notice.

----------------------------------
>From 3fd3bc58dc6505af73ecf92c981609ecf8b6ac40 Mon Sep 17 00:00:00 2001
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Date: Fri, 13 Nov 2009 16:52:03 +0900
Subject: [PATCH] [RFC] mm: non linear mapping page don't mark as PG_mlocked

Now, try_to_unmap_file() lost the capability to treat VM_NONLINEAR.
Then, mlock() shouldn't mark the page of non linear mapping as
PG_mlocked. Otherwise the page continue to drinker walk between
evictable and unevictable lru.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
mm/mlock.c | 37 +++++++++++++++++++++++--------------
1 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 48691fb..4187f9c 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -266,25 +266,34 @@ long mlock_vma_pages_range(struct vm_area_struct *vma,
if (vma->vm_flags & (VM_IO | VM_PFNMAP))
goto no_mlock;

- if (!((vma->vm_flags & (VM_DONTEXPAND | VM_RESERVED)) ||
+ if ((vma->vm_flags & (VM_DONTEXPAND | VM_RESERVED)) ||
is_vm_hugetlb_page(vma) ||
- vma == get_gate_vma(current))) {
+ vma == get_gate_vma(current)) {
+
+ /*
+ * User mapped kernel pages or huge pages:
+ * make these pages present to populate the ptes, but
+ * fall thru' to reset VM_LOCKED--no need to unlock, and
+ * return nr_pages so these don't get counted against task's
+ * locked limit. huge pages are already counted against
+ * locked vm limit.
+ */
+ make_pages_present(start, end);
+ goto no_mlock;
+ }

+ if (vma->vm_flags & VM_NONLINEAR)
+ /*
+ * try_to_munmap() doesn't treat VM_NONLINEAR. let's make
+ * consist.
+ */
+ make_pages_present(start, end);
+ else
__mlock_vma_pages_range(vma, start, end);

- /* Hide errors from mmap() and other callers */
- return 0;
- }
+ /* Hide errors from mmap() and other callers */
+ return 0;

- /*
- * User mapped kernel pages or huge pages:
- * make these pages present to populate the ptes, but
- * fall thru' to reset VM_LOCKED--no need to unlock, and
- * return nr_pages so these don't get counted against task's
- * locked limit. huge pages are already counted against
- * locked vm limit.
- */
- make_pages_present(start, end);

no_mlock:
vma->vm_flags &= ~VM_LOCKED; /* and don't come back! */
--
1.6.2.5