From: Jason Gunthorpe <jgg@ziepe.ca>
To: Steven Sistare <steven.sistare@oracle.com>
Cc: Alex Williamson <alex.williamson@redhat.com>,
lizhe.67@bytedance.com, cohuck@redhat.com, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, lizefan.x@bytedance.com
Subject: Re: [PATCH] vfio: remove useless judgement
Date: Tue, 28 Jun 2022 11:04:59 -0300 [thread overview]
Message-ID: <20220628140459.GP23621@ziepe.ca> (raw)
In-Reply-To: <f7b7a7b0-9404-6b0f-99b5-346af041a479@oracle.com>
On Tue, Jun 28, 2022 at 09:54:19AM -0400, Steven Sistare wrote:
> >> As you and I have discussed, the count is also wrong in the direct
> >> exec model, because exec clears mm->locked_vm.
> >
> > Really? Yikes, I thought exec would generate a new mm?
>
> Yes, exec creates a new mm with locked_vm = 0. The old locked_vm count is dropped
> on the floor. The existing dma points to the same task, but task->mm has changed,
> and dma->task->mm->locked_vm is 0. An unmap ioctl drives it
> negative.
Oh.. This is probably a bug, vfio should never use task->mm, the mm
itself should be held using mmgrab instead.
Otherwise exec case is broken as you describe.
> I have prototyped a few possible fixes. One changes vfio to use user->locked_vm.
> Another changes to mm->pinned_vm and preserves it during exec. A third preserves
> mm->locked_vm across exec, but that is not practical, because mm->locked_vm mixes
> vfio pins and mlocks. The mlock component must be cleared during exec, and we don't
> have a separate count for it.
Lossing locked_vm on exec/fork is the correct and expected behavior
for the core kernel code, the bug is that vfio drives it negative.
Jason
next prev parent reply other threads:[~2022-06-28 14:05 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-27 3:51 [PATCH] vfio: remove useless judgement lizhe.67
2022-06-27 22:06 ` Alex Williamson
2022-06-28 12:48 ` Steven Sistare
2022-06-28 13:03 ` Jason Gunthorpe
2022-06-28 13:54 ` Steven Sistare
2022-06-28 14:04 ` Jason Gunthorpe [this message]
2022-06-30 19:51 ` Alex Williamson
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