Date: Wed, 17 May 2023 09:29:20 +0200
From: Jan Kara <jack@...e.cz>
To: Lorenzo Stoakes <lstoakes@...il.com>
Cc: Jason Gunthorpe <jgg@...dia.com>,
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Subject: Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default

On Mon 15-05-23 14:07:57, Lorenzo Stoakes wrote:
> On Mon, May 15, 2023 at 09:12:49AM -0300, Jason Gunthorpe wrote:
> > On Mon, May 15, 2023 at 12:16:21PM +0100, Lorenzo Stoakes wrote:
> > > Jason will have some thoughts on this I'm sure. I guess the key question
> > > here is - is it actually feasible for this to work at all? Once we
> > > establish that, the rest are details :)
> >
> > Surely it is, but like Ted said, the FS folks are not interested and
> > they are at least half the solution..
> 
> :'(

Well, I'd phrase this a bit differently - it is a difficult sell to fs
maintainers that they should significantly complicate writeback code / VFS
with bounce page handling etc. for a thing that is not much used corner
case. So if we can get away with forbiding long-term pins, then that's the
easiest solution. Dealing with short-term pins is easier as we can just
wait for unpinning which is implementable in a localized manner.

> > The FS also has to actively not write out the page while it cannot be
> > write protected unless it copies the data to a stable page. The block
> > stack needs the source data to be stable to do checksum/parity/etc
> > stuff. It is a complicated subject.
> 
> Yes my sense was that being able to write arbitrarily to these pages _at
> all_ was a big issue, not only the dirty tracking aspect.

Yes.

> I guess at some level letting filesystems have such total flexibility as to
> how they implement things leaves us in a difficult position.

I'm not sure what you mean by "total flexibility" here. In my opinion it is
also about how HW performs checksumming etc.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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