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    SubjectRe: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial
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    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@huawei.com> 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

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