linux-kernel - Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <d17c0fce-679b-4f5d-9a7c-6ff7e28ad4b2@lucifer.local>
Date:   Wed, 17 May 2023 08:40:26 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Matthew Wilcox <willy@...radead.org>,
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        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default

On Wed, May 17, 2023 at 09:29:20AM +0200, Jan Kara wrote:
> 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.
>

Totally understandable. It's unfortunately I feel a case of something we
should simply not have allowed.

> > > 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.

I mean to say *_ops allow a lot of flexibility in how things are
handled. Certainly checksumming is a great example but in theory an
arbitrary filesystem could be doing, well, anything and always assuming
that only userland mappings should be modifying the underlying data.

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