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.
They are ways to minimize the impact (for instance by using the
mmu notifier type to ignore some invalidation cases).
So i would like to discuss all this during LSF, it is mostly a
filesystem discussion with strong tie to mm.
[1] GUP https://lkml.org/lkml/2019/3/8/805 and all subsequent
discussion.
To: lsf-pc@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Benjamin LaHaise <bcrl@xxxxxxxxx>
