Re: [PATCH 0/3] ksm: write protect pages from inside ksm

From: Izik Eidus
Date: Sun Jun 14 2009 - 18:16:35 EST

Hugh Dickins wrote:
On Sat, 13 Jun 2009, Izik Eidus wrote:
Hugh, so untill here we are sync,

Yes, that fits with what I have here, thanks (or where it didn't
quite fit, e.g. ' versus `, I've adjusted to what you have!). And
thanks for fixing my *orig_pte = *ptep bug, you did point that out
before, but I misunderstood at first.

Question is what you want me to do now?,
(Beacuse we are skipping 2.6.31, It is ok to you to tell me something
like: "Shut up and let me see what i can get with this madvise" -
that from one side.
From another side if you want me to do anything please say.

I had to get a bit further at my end before answering on that,
but now the answer is clear: please do some testing of your RFC
madvise() version (which is what I'm just tidying up a little),
and let me know any bugfixes you find. Try with SLAB or SLUB or
SLQB debug on e.g. CONFIG_SLUB=y, CONFIG_SLUB_DEBUG=y and boot
option "slub_debug".

Sure, let me check it.
(You do have Andrea patch that fix the "used after free slab entries" ?)

I'm finding, whether with your RFC or my tidyup, that kksmd
soon oopses in get_next_mmlist (or perhaps find_vma): presumably
accessing a vma or mm which already got freed (if you don't have
slab debugging on, it's liable to hang instead).

(I've also not seen it actually merging yet: if you register
or madvise a large anon area and memset it, the /dev/ksm version
would merge all its pages, but I've not seen the madvise version
do so yet - though maybe there's something stupidly wrong in my
testing, really I'm more worried about the oopses at present.)

Note that mmotm includes a patch of Nick's which adds a function
madvise_behavior_valid() - you'll need to add your MADVs into its
list to get it to work at all there.

Here's a patch I added a month or so ago, when trying to experiment
with KSM on all mms: shouldn't be necessary if your mm refcounting
is right, but might help to avoid extra weirdness when things go
wrong: exit_mmap() leaves stale vma pointers around, reckoning
that nobody can be interested by now; but maybe KSM might peep
so better to tidy them up at least while debugging...


--- old/mm/mmap.c 2009-05-01 13:47:45.000000000 +0100
+++ new/mm/mmap.c 2009-05-03 11:34:47.000000000 +0100
@@ -2112,6 +2112,14 @@ void exit_mmap(struct mm_struct *mm)
tlb_finish_mmu(tlb, 0, end);
+ * Make sure get_user_pages() and find_vma() etc. will find nothing:
+ * this may be necessary for KSM.
+ */
+ mm->mmap = NULL;
+ mm->mmap_cache = NULL;
+ mm->mm_rb = RB_ROOT;
+ /*
* Walk the list again, actually closing and freeing it,
* with preemption enabled, without holding any MM locks.

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