[PATCH 0/4] Volatile Ranges (v14 - madvise reborn edition!)

John Stultz
2014-04-29T21:21:39Z

Another few weeks and another volatile ranges patchset...

After getting the sense that the a major objection to the earlier
patches was the introduction of a new syscall (and its somewhat
strange dual length/purged-bit return values), I spent some time
trying to rework the vma manipulations so we can be we won't fail
mid-way through changing volatility (basically making it atomic).
I think I have it working, and thus, there is no longer the
need for a new syscall, and we can go back to using madvise()
to set and unset pages as volatile.

New changes are:
----------------
o Reworked vma manipulations to be be atomic
o Converted back to using madvise() as syscall interface
o Integrated fix from Minchan to avoid SIGBUS faulting race
o Caught/fixed subtle use-after-free bug w/ vma merging
o Lots of minor cleanups and comment improvements

Still on the TODO list
----------------------------------------------------
o Sort out how best to do page accounting when the volatility
is tracked on a per-mm basis.
o Revisit anonymous page aging on swapless systems
o Draft up re-adding tmpfs/shm file volatility support

Many thanks again to Minchan, Kosaki-san, Johannes, Jan, Rik,
Hugh, and others for the great feedback and discussion at
LSF-MM.

thanks
-john

Volatile ranges provides a method for userland to inform the kernel
that a range of memory is safe to discard (ie: can be regenerated)
but userspace may want to try access it in the future. It can be
thought of as similar to MADV_DONTNEED, but that the actual freeing
of the memory is delayed and only done under memory pressure, and the
user can try to cancel the action and be able to quickly access any
unpurged pages. The idea originated from Android's ashmem, but I've
since learned that other OSes provide similar functionality.

This functionality allows for a number of interesting uses. One such
example is: Userland caches that have kernel triggered eviction under
memory pressure. This allows for the kernel to "rightsize" userspace
caches for current system-wide workload. Things like image bitmap
caches, or rendered HTML in a hidden browser tab, where the data is
not visible and can be regenerated if needed, are good examples.

Both Chrome and Firefox already make use of volatile range-like
functionality via the ashmem interface:
https://hg.mozilla.org/releases/mozilla-b2g28_v1_3t/rev/a32c32b24a34

https://chromium.googlesource.com/chromium/src/base/+/47617a69b9a57796935e03d78931bd01b4806e70/memory/discardable_memory_allocator_android.cc

The basic usage of volatile ranges is as so:
1) Userland marks a range of memory that can be regenerated if
necessary as volatile
2) Before accessing the memory again, userland marks the memory as
nonvolatile, and the kernel will provide notification if any pages in
the range has been purged.

If userland accesses memory while it is volatile, it will either
get the value stored at that memory if there has been no memory
pressure or the application will get a SIGBUS if the page has been
purged.

Reads or writes to the memory do not affect the volatility state of the
pages.

You can read more about the history of volatile ranges here (~reverse
chronological order):
https://lwn.net/Articles/592042/
https://lwn.net/Articles/590991/
http://permalink.gmane.org/gmane.linux.kernel.mm/98848
http://permalink.gmane.org/gmane.linux.kernel.mm/98676
https://lwn.net/Articles/522135/
https://lwn.net/Kernel/Index/#Volatile_ranges

Continuing from the last few releases, this revision is reduced in
scope when compared to earlier attempts. I've only focused on handled
volatility on anonymous memory, and we're storing the volatility in
the VMA. This may have performance implications compared with the
earlier approach, but it does simplify the approach. I'm open to
expanding functionality via flags arguments, but for now I'm wanting
to keep focus on what the right default behavior should be and keep
the use cases restricted to help get reviewer interest.

Additionally, since we don't handle volatility on tmpfs files with this
version of the patch, it is not able to be used to implement semantics
similar to Android's ashmem. But since shared volatiltiy on files is
more complex, my hope is to start small and hopefully grow from there.

Again, much of the logic in this patchset is based on Minchan's earlier
efforts, so I do want to make sure the credit goes to him for his major
contribution!

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Android Kernel Team <kernel-team@android.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Robert Love <rlove@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Rik van Riel <riel@redhat.com>
Cc: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Hommey <mh@glandium.org>
Cc: Taras Glek <tglek@mozilla.com>
Cc: Jan Kara <jack@suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Keith Packard <keithp@keithp.com>
Cc: linux-mm@kvack.org <linux-mm@kvack.org>

John Stultz (4):
swap: Cleanup how special swap file numbers are defined
MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks and handle marking
vmas
MADV_VOLATILE: Add purged page detection on setting memory
non-volatile
MADV_VOLATILE: Add page purging logic & SIGBUS trap

include/linux/mm.h | 1 +
include/linux/mvolatile.h | 7 +
include/linux/swap.h | 36 +++-
include/linux/swapops.h | 10 +
include/uapi/asm-generic/mman-common.h | 5 +
mm/Makefile | 2 +-
mm/internal.h | 2 -
mm/madvise.c | 14 ++
mm/memory.c | 7 +
mm/mvolatile.c | 353 +++++++++++++++++++++++++++++++++
mm/rmap.c | 5 +
mm/vmscan.c | 12 ++
12 files changed, 440 insertions(+), 14 deletions(-)
create mode 100644 include/linux/mvolatile.h
create mode 100644 mm/mvolatile.c

--
1.9.1