Messages in this thread | | | Subject | Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial | From | Wengang Wang <> | Date | Tue, 20 Nov 2018 10:10:31 -0800 |
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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@oracle.com> >> --- >> 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; >> >
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