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:

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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.

Full agreement here.  The whole concept of supporting writeback for
long term mappings does not make much sense.

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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.

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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 have no idea what total flexbility is even supposed to be.