linux-kernel - Re: WARNING in _

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Message-ID: <025aa12a-c789-7eac-ba96-48e4dd3dd551@redhat.com>
Date:   Tue, 23 Jul 2019 21:37:23 +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 2019/7/23 下午6:42, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 04:42:19PM +0800, Jason Wang wrote:
>>> So how about this: do exactly what you propose but as a 2 patch series:
>>> start with the slow safe patch, and add then return uaddr optimizations
>>> on top. We can then more easily reason about whether they are safe.
>>
>> If you stick, I can do this.
> So I definitely don't insist but I'd like us to get back to where
> we know existing code is very safe (if not super fast) and
> optimizing from there.  Bugs happen but I'd like to see a bisect
> giving us "oh it's because of XYZ optimization" and not the
> general "it's somewhere within this driver" that we are getting
> now.


Syzbot has bisected to the commit of metadata acceleration in fact :)


>
> Maybe the way to do this is to revert for this release cycle
> and target the next one. What do you think?


I would try to fix the issues consider packed virtqueue which may use 
this for a good performance number. But if you insist, I'm ok to revert. 
Or maybe introduce a config option to disable it by default (almost all 
optimized could be ruled out).

Thanks