linux-kernel - Re: [PATCH] vfio: remove useless judgement

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Message-ID: <20220628140459.GP23621@ziepe.ca>
Date:   Tue, 28 Jun 2022 11:04:59 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        lizhe.67@...edance.com, cohuck@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, lizefan.x@...edance.com
Subject: Re: [PATCH] vfio: remove useless judgement

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