Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

[Christoph Hellwig <hch@lst.de>]

On Sun, Jun 17, 2018 at 01:28:18PM -0700, John Hubbard wrote:
> Yes. However, my thinking was: get_user_pages() can become a way to indicate that 
> these pages are going to be treated specially. In particular, the caller
> does not really want or need to support certain file operations, while the
> page is flagged this way.
> 
> If necessary, we could add a new API call.

That API call is called get_user_pages_longterm.

> But either way, I think we could
> reasonably document that "if you pin these pages (either via get_user_pages,
> or some new, similar-looking API call), you can DMA to/from them, and safely
> mark them as dirty when you're done, and the right things will happen. 
> And in the interim, you can expect that the follow file system API calls
> will not behave predictably: fallocate, truncate, ..."

That is not how get_user_pages(_fast) is used.  We use it all over the
kernel, including for direct I/O.  You'd break a lot of existing use
cases very badly.