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Date:   Fri, 28 Apr 2023 14:31:38 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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On Fri, Apr 28, 2023 at 07:02:22PM +0200, David Hildenbrand wrote:

> > No. VMA cannot get away before PTEs are unmapped and TLB is flushed. And
> > TLB flushing is serialized against GUP_fast().
>
> The whole CONFIG_MMU_GATHER_RCU_TABLE_FREE handling makes the situation more
> complicated.

Yeah, you have to think of gup_fast as RCU with a hacky pre-RCU implementation
on most architectures.

We could make page->mapping safe under RCU, for instance.

Jason