Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial

[Wengang Wang]

Hi Wei,

I think you will receive my reply to Zhong, But I am copying my comments 
for that patch here (again):

Copy starts ==>

I am not sure if the patch you mentioned intended to fix the problem here.
With that patch the negative page->pobjects would become a large 
positive value,
it will win the compare with s->cpu_partial and go ahead to unfreeze the 
partial slabs.
Though it may be not a perfect fix for this issue, it really fixes (or 
workarounds) the issue here.
I'd like to skip my patch..

<=== Copy ends

thanks,

wengang


On 2018/11/20 19:02, Wei Yang wrote:
> On Tue, Nov 20, 2018 at 09:58:58AM -0800, Wengang Wang wrote:
> > Hi Wei,
> >
> >
> > On 2018/11/17 17:02, Wei Yang wrote:
> > > On Fri, Nov 16, 2018 at 05:33:35PM -0800, Wengang Wang wrote:
> > > > The this_cpu_cmpxchg makes the do-while loop pass as long as the
> > > > s->cpu_slab->partial as the same value. It doesn't care what happened to
> > > > that slab. Interrupt is not disabled, and new alloc/free can happen in the
> > > Well, I seems to understand your description.
> > >
> > > There are two slabs
> > >
> > >      * one which put_cpu_partial() trying to free an object
> > >      * one which is the first slab in cpu_partial list
> > >
> > > There is some tricky case, the first slab in cpu_partial list we
> > > reference to will change since interrupt is not disabled.
> > Yes, two slabs involved here just as you said above.
> > And yes, the case is really tricky, but it's there.
> >
> > > > interrupt handlers. Theoretically, after we have a reference to the it,
> > >                                                                    ^^^
> > > 							 one more word?
> > sorry, "the" should not be there.
> >
> > > > stored in _oldpage_, the first slab on the partial list on this CPU can be
> > >                                               ^^^
> > > One little suggestion here, mayby use cpu_partial would be more easy to
> > > understand. I confused this with the partial list in kmem_cache_node at
> > > the first time.  :-)
> > Right, making others understanding easily is very important. I just meant
> > cpu_partial.
> >
> > > > moved to kmem_cache_node and then moved to different kmem_cache_cpu and
> > > > then somehow can be added back as head to partial list of current
> > > > kmem_cache_cpu, though that is a very rare case. If that rare case really
> > > Actually, no matter what happens after the removal of the first slab in
> > > cpu_partial, it would leads to problem.
> > Maybe you are right, what I see is the problem on the page->pobjects.
> >
> > > > happened, the reading of oldpage->pobjects may get a 0xdead0000
> > > > unexpectedly, stored in _pobjects_, if the reading happens just after
> > > > another CPU removed the slab from kmem_cache_node, setting lru.prev to
> > > > LIST_POISON2 (0xdead000000000200). The wrong _pobjects_(negative) then
> > > > prevents slabs from being moved to kmem_cache_node and being finally freed.
> > > >
> > > > We see in a vmcore, there are 375210 slabs kept in the partial list of one
> > > > kmem_cache_cpu, but only 305 in-use objects in the same list for
> > > > kmalloc-2048 cache. We see negative values for page.pobjects, the last page
> > > > with negative _pobjects_ has the value of 0xdead0004, the next page looks
> > > > good (_pobjects is 1).
> > > >
> > > > For the fix, I wanted to call this_cpu_cmpxchg_double with
> > > > oldpage->pobjects, but failed due to size difference between
> > > > oldpage->pobjects and cpu_slab->partial. So I changed to call
> > > > this_cpu_cmpxchg_double with _tid_. I don't really want no alloc/free
> > > > happen in between, but just want to make sure the first slab did expereince
> > > > a remove and re-add. This patch is more to call for ideas.
> > > Maybe not an exact solution.
> > >
> > > I took a look into the code and change log.
> > >
> > > _tid_ is introduced by commit 8a5ec0ba42c4 ('Lockless (and preemptless)
> > > fastpaths for slub'), which is used to guard cpu_freelist. While we don't
> > > modify _tid_ when cpu_partial changes.
> > >
> > > May need another _tid_ for cpu_partial?
> > Right, _tid_ changes later than cpu_partial changes.
> >
> > As pointed out by Zhong Jiang, the pobjects issue is fixed by commit
> Where you discussed this issue? Any reference I could get a look?
>
> > e5d9998f3e09 (not sure if by side effect, see my replay there),
> I took a look at this commit e5d9998f3e09 ('slub: make ->cpu_partial
> unsigned int'), but not see some relationship between them.
>
> Would you mind show me a link or cc me in case you have further
> discussion?
>
> Thanks.
>
> > I'd skip this patch.?? If we found other problems regarding the change of
> > cpu_partial, let's fix them. What do you think?
> >
> > thanks,
> > wengang