Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial

[Wengang Wang]

Hi Zhong,


On 2018/11/19 18:18, zhong jiang wrote:
> On 2018/11/17 9:33, 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
> > interrupt handlers. Theoretically, after we have a reference to the it,
> > stored in _oldpage_, the first slab on the partial list on this CPU can be
> > 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
> > 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.
> Have you hit the really issue or just review the code ?
Yup, I hit the real issue. The root cause is out by reviewing the code.

> I did hit the issue and fixed in the upstream patch unpredictably by the following patch.
> e5d9998f3e09 ("slub: make ->cpu_partial unsigned int")
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..

thanks,
wengang

> Thanks,
> zhong jiang
> > Signed-off-by: Wengang Wang <wen.gang.wang@xxxxxxxxxx>
> > ---
> >   mm/slub.c | 20 +++++++++++++++++---
> >   1 file changed, 17 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index e3629cd..26539e6 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2248,6 +2248,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> >   {
> >   #ifdef CONFIG_SLUB_CPU_PARTIAL
> >   	struct page *oldpage;
> > +	unsigned long tid;
> >   	int pages;
> >   	int pobjects;
> >   
> > @@ -2255,8 +2256,12 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> >   	do {
> >   		pages = 0;
> >   		pobjects = 0;
> > -		oldpage = this_cpu_read(s->cpu_slab->partial);
> >   
> > +		tid = this_cpu_read(s->cpu_slab->tid);
> > +		/* read tid before reading oldpage */
> > +		barrier();
> > +
> > +		oldpage = this_cpu_read(s->cpu_slab->partial);
> >   		if (oldpage) {
> >   			pobjects = oldpage->pobjects;
> >   			pages = oldpage->pages;
> > @@ -2283,8 +2288,17 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> >   		page->pobjects = pobjects;
> >   		page->next = oldpage;
> >   
> > -	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
> > -								!= oldpage);
> > +		/* we dont' change tid, but want to make sure it didn't change
> > +		 * in between. We don't really hope alloc/free not happen on
> > +		 * this CPU, but don't want the first slab be removed from and
> > +		 * then re-added as head to this partial list. If that case
> > +		 * happened, pobjects may read 0xdead0000 when this slab is just
> > +		 * removed from kmem_cache_node by other CPU setting lru.prev
> > +		 * to LIST_POISON2.
> > +		 */
> > +	} while (this_cpu_cmpxchg_double(s->cpu_slab->partial, s->cpu_slab->tid,
> > +					 oldpage, tid, page, tid) == 0);
> > +
> >   	if (unlikely(!s->cpu_partial)) {
> >   		unsigned long flags;
> >