Re: Does GUP page unpinning have to be done in the pinning context?

[Christoph Hellwig <hch@infradead.org>]

On Wed, Apr 09, 2025 at 07:56:07PM -0700, John Hubbard wrote:
> This topic always worries me, because the original problem with
> dirty pages is still unfixed: setting pages dirty upon unpinning
> is both widely done (last time I checked), and yet broken, because
> it doesn't do a mkdirty() call to set up writeback buffers.
> 
> The solution always seemed to point toward "get a file lease on that
> range, before pinning", but it's a contentious design area to say
> the least.

For the bio based direct I/O implementations we do set the pages
dirty before starting I/O using bio_set_pages_dirty, which uses
folio_mark_dirty and thus calls into the file systems using
->dirty_folio.  But we also do a second pass on I/O completion
before the buffers are unpinned.  Which I think now that we pin
the folios is superfluous.