Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial

[Wengang Wang]

On 2018/11/26 16:36, Wei Yang wrote:
> On Mon, Nov 26, 2018 at 08:57:54AM -0800, Wengang Wang wrote:
> > On 2018/11/25 17:59, Wei Yang wrote:
> > > On Tue, Nov 20, 2018 at 10:58 AM zhong jiang <zhongjiang@xxxxxxxxxx> 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 ?
> > > >
> > > > I did hit the issue and fixed in the upstream patch unpredictably by the following patch.
> > > > e5d9998f3e09 ("slub: make ->cpu_partial unsigned int")
> > > Zhong,
> > >
> > > I took a look into your upstream patch, while I am confused how your patch
> > > fix this issue?
> > >
> > > In put_cpu_partial(), the cmpxchg compare cpu_slab->partial (a page struct)
> > > instead of the cpu_partial (an unsigned integer). I didn't get the
> > > point of this fix.
> > I think the patch can't prevent pobjects from being set as 0xdead0000 (the
> > primary 4 bytes of LIST_POISON2).
> > But if pobjects is treated as unsigned integer,
> >
> > 2266???????????????????????????????????????????????? pobjects = oldpage->pobjects;
> > 2267???????????????????????????????????????????????? pages = oldpage->pages;
> > 2268???????????????????????????????????????????????? if (drain && pobjects > s->cpu_partial) {
> > 2269???????????????????????????????????????????????????????????????? unsigned long flags;
> Ehh..., you mean (0xdead0000 > 0x02) ?
Yes.

> This is really a bad thing, if it wordarounds the problem like this.
It does.
> I strongly don't agree this is a *fix*. This is too tricky.
It's tricky. I don't know for what purpose the patch went in. The commit 
message was just saying _pobjects_ should be "unsigned"...

thanks,
wengang

> > line 2268 will be true in put_cpu_partial(), thus code goes to
> > unfreeze_partials(). This way the slabs in the cpu partial list can be moved
> > to kmem_cache_nod and then freed. So it fixes (or say workarounds) the
> > problem I see here (huge number of empty slabs stay in cpu partial list).
> >
> > thanks
> > wengang
> >
> > > > Thanks,
> > > > zhong jiang