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@xxxxxxxxxxxxxxxxxxxx> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Robert Love <rlove@xxxxxxxxxx> Cc: Mel Gorman <mel@xxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Dave Hansen <dave@xxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx> Cc: Neil Brown <neilb@xxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Mike Hommey <mh@xxxxxxxxxxxx> Cc: Taras Glek <tglek@xxxxxxxxxxx> Cc: Jan Kara <jack@xxxxxxx> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> Cc: Michel Lespinasse <walken@xxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Keith Packard <keithp@xxxxxxxxxx> Cc: linux-mm@xxxxxxxxx <linux-mm@xxxxxxxxx> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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