linux-kernel - Re: WARNING in _

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Date:   Mon, 29 Jul 2019 13:56:08 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        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11:03, Jason Gunthorpe wrote:
> On Fri, Jul 26, 2019 at 10:00:20PM +0800, Jason Wang wrote:
>> The question is, MMU notifier are allowed to be blocked on
>> invalidate_range_start() which could be much slower than synchronize_rcu()
>> to finish.
>>
>> Looking at amdgpu_mn_invalidate_range_start_gfx() which calls
>> amdgpu_mn_invalidate_node() which did:
>>
>>                  r = reservation_object_wait_timeout_rcu(bo->tbo.resv,
>>                          true, false, MAX_SCHEDULE_TIMEOUT);
>>
>> ...
> The general guidance has been that invalidate_start should block
> minimally, if at all.
>
> I would say synchronize_rcu is outside that guidance.


Yes, I get this.


>
> BTW, always returning EAGAIN for mmu_notifier_range_blockable() is not
> good either, it should instead only return EAGAIN if any
> vhost_map_range_overlap() is true.


Right, let me optimize that.

Thanks


>
> Jason