linux-kernel - Re: [PATCH] vfio/pci: Handle concurrent vma faults

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Message-ID: <20210313000306.GI2356281@nvidia.com>
Date:   Fri, 12 Mar 2021 20:03:06 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, prime.zeng@...ilicon.com, cohuck@...hat.com
Subject: Re: [PATCH] vfio/pci: Handle concurrent vma faults

On Fri, Mar 12, 2021 at 01:58:44PM -0700, Alex Williamson wrote:

> Yeah, we can indeed use memalloc_nofs_save/restore().  It seems we're
> trying to allocate something for pfnmap tracking and that enables lots
> of lockdep specific tests.  Is it valid to wrap io_remap_pfn_range()
> around clearing this flag or am I just masking a bug?  Thanks,

Yes, I think it is fine. Those functions are ment to be used in a
no-fs kind of region exactly like this.

no-fs is telling the allocator not to do reclaim which is forbidden
under the locks here (as reclaim will also attempt to get these locks)

I would defer to Michal Hocko though, maybe cc him on the final patch
series version.

Jason