linux-kernel - Re: WARNING in _

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Message-ID: <20190726150322.GB8695@ziepe.ca>
Date:   Fri, 26 Jul 2019 12:03:22 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jason Wang <jasowang@...hat.com>
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 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.

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.

Jason