linux-kernel - Re: WARNING in _

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Date:   Mon, 29 Jul 2019 13:54:49 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@...kaller.appspotmail.com>,
        aarcange@...hat.com, akpm@...ux-foundation.org,
        christian@...uner.io, davem@...emloft.net, ebiederm@...ssion.com,
        elena.reshetova@...el.com, guro@...com, hch@...radead.org,
        james.bottomley@...senpartnership.com, jglisse@...hat.com,
        keescook@...omium.org, ldv@...linux.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-parisc@...r.kernel.org,
        luto@...capital.net, mhocko@...e.com, mingo@...nel.org,
        namit@...are.com, peterz@...radead.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        wad@...omium.org
Subject: Re: WARNING in __mmdrop


On 2019/7/26 下午9:49, Michael S. Tsirkin wrote:
>>> Ok, let me retry if necessary (but I do remember I end up with deadlocks
>>> last try).
>> Ok, I play a little with this. And it works so far. Will do more testing
>> tomorrow.
>>
>> One reason could be I switch to use get_user_pages_fast() to
>> __get_user_pages_fast() which doesn't need mmap_sem.
>>
>> Thanks
> OK that sounds good. If we also set a flag to make
> vhost_exceeds_weight exit, then I think it will be all good.


After some experiments, I came up two methods:

1) switch to use vq->mutex, then we must take the vq lock during range 
checking (but I don't see obvious slowdown for 16vcpus + 16queues). 
Setting flags during weight check should work but it still can't address 
the worst case: wait for the page to be swapped in. Is this acceptable?

2) using current RCU but replace synchronize_rcu() with 
vhost_work_flush(). The worst case is the same as 1) but we can check 
range without holding any locks.

Which one did you prefer?

Thanks