On Tue, Jan 21, 2020 at 6:34 PM <jglisse@xxxxxxxxxx> wrote: > > From: Jérôme Glisse <jglisse@xxxxxxxxxx> > > Direct I/O does pin memory through GUP (get user page) this does > block several mm activities like: > - compaction > - numa > - migration > ... > > It is also troublesome if the pinned pages are actualy file back > pages that migth go under writeback. In which case the page can > not be write protected from direct-io point of view (see various > discussion about recent work on GUP [1]). This does happens for > instance if the virtual memory address use as buffer for read > operation is the outcome of an mmap of a regular file. > > > With direct-io or aio (asynchronous io) pages are pinned until > syscall completion (which depends on many factors: io size, > block device speed, ...). For io-uring pages can be pinned an > indifinite amount of time. > > > So i would like to convert direct io code (direct-io, aio and > io-uring) to obey mmu notifier and thus allow memory management > and writeback to work and behave like any other process memory. > > For direct-io and aio this mostly gives a way to wait on syscall > completion. For io-uring this means that buffer might need to be > re-validated (ie looking up pages again to get the new set of > pages for the buffer). Impact for io-uring is the delay needed > to lookup new pages or wait on writeback (if necessary). This > would only happens _if_ an invalidation event happens, which it- > self should only happen under memory preissure or for NUMA > activities. This seems to assume that memory pressure and NUMA migration are rare events. Some of the proposed hierarchical memory management schemes [1] might impact that assumption. [1]: http://lore.kernel.org/r/20191101075727.26683-1-ying.huang@xxxxxxxxx/
