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

[Jason Gunthorpe]

On Mon, Jul 09, 2018 at 09:47:40PM +0200, Jan Kara wrote:
> On Mon 09-07-18 10:16:51, Matthew Wilcox wrote:
> > On Mon, Jul 09, 2018 at 06:08:06PM +0200, Jan Kara wrote:
> > > On Mon 09-07-18 18:49:37, Nicholas Piggin wrote:
> > > > The problem with blocking in clear_page_dirty_for_io is that the fs is
> > > > holding the page lock (or locks) and possibly others too. If you
> > > > expect to have a bunch of long term references hanging around on the
> > > > page, then there will be hangs and deadlocks everywhere. And if you do
> > > > not have such log term references, then page lock (or some similar lock
> > > > bit) for the duration of the DMA should be about enough?
> > > 
> > > There are two separate questions:
> > > 
> > > 1) How to identify pages pinned for DMA? We have no bit in struct page to
> > > use and we cannot reuse page lock as that immediately creates lock
> > > inversions e.g. in direct IO code (which could be fixed but then good luck
> > > with auditing all the other GUP users). Matthew had an idea and John
> > > implemented it based on removing page from LRU and using that space in
> > > struct page. So we at least have a way to identify pages that are pinned
> > > and can track their pin count.
> > > 
> > > 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 think as a userspace I would expect the 'current content' to be
flushed without waiting..

If you block fsync() then anyone using a RDMA MR with it will just
dead lock. What happens if two processes open the same file and
one makes a MR and the other calls fsync()? Sounds bad.

Jason