Re: [PATCH 0/2] mm/fs: put_user_page() proposal

[Matthew Wilcox]

On Mon, Jul 09, 2018 at 09:47:40PM +0200, Jan Kara wrote:
> On Mon 09-07-18 10:16:51, Matthew Wilcox wrote:
> > > 2) What to do when some page is pinned but we need to do e.g.
> > > clear_page_dirty_for_io(). After some more thinking I agree with you that
> > > just blocking waiting for page to unpin will create deadlocks like:
> > 
> > Why are we trying to writeback a page that is pinned?  It's presumed to
> > be continuously redirtied by its pinner.  We can't evict it.
> 
> So what should be a result of fsync(file), where some 'file' pages are
> pinned e.g. by running direct IO? If we just skip those pages, we'll lie to
> userspace that data was committed while it was not (and it's not only about
> data that has landed in those pages via DMA, you can have first 1k of a page
> modified by normal IO in parallel to DMA modifying second 1k chunk). If
> fsync(2) returns error, it would be really unexpected by userspace and most
> apps will just not handle that correctly. So what else can you do than
> block?

I was thinking about writeback, and neglected the fsync case.  For fsync,
we could copy the "current" contents of the page to a freshly-allocated
page and write _that_ to disc?  As long as we redirty the real page after
the pin is dropped, I think we're fine.