|From:||Dan Magenheimer <dan.magenheimer-AT-oracle.com>|
|To:||linux-kernel-AT-vger.kernel.org, linux-mm-AT-kvack.org, jeremy-AT-goop.org, hugh.dickins-AT-tiscali.co.uk, ngupta-AT-vflare.org, JBeulich-AT-novell.com, chris.mason-AT-oracle.com, kurt.hackel|
|Subject:||Frontswap [PATCH 0/4] (was Transcendent Memory): overview|
|Date:||Thu, 22 Apr 2010 06:42:49 -0700|
Frontswap [PATCH 0/4] (was Transcendent Memory): overview Patch applies to 2.6.34-rc5 In previous patch postings, frontswap was part of the Transcendent Memory ("tmem") patchset. This patchset refocuses not on the underlying technology (tmem) but instead on the useful functionality provided for Linux, and provides a clean API so that frontswap can provide this very useful functionality via a Xen tmem driver OR completely independent of tmem. For example: Nitin Gupta (of compcache and ramzswap fame) is implementing an in-kernel compression "backend" for frontswap; some believe frontswap will be a very nice interface for building RAM-like functionality for pseudo-RAM devices such as SSD or phase-change memory; and a Pune University team is looking at a backend for virtio (see OLS'2010). A more complete description of frontswap can be found in the introductory comment in mm/frontswap.c (in PATCH 2/4) which is included below for convenience. Note that an earlier version of this patch is now shipping in OpenSuSE 11.2 and will soon ship in a release of Oracle Enterprise Linux. Underlying tmem technology is now shipping in Oracle VM 2.2 and was just released in Xen 4.0 on April 15, 2010. (Search news.google.com for Transcedent Memory) Signed-off-by: Dan Magenheimer <firstname.lastname@example.org> Reviewed-by: Jeremy Fitzhardinge <email@example.com> include/linux/frontswap.h | 98 ++++++++++++++ include/linux/swap.h | 2 include/linux/swapfile.h | 13 + mm/Kconfig | 16 ++ mm/Makefile | 1 mm/frontswap.c | 301 ++++++++++++++++++++++++++++++++++++++++++++++ mm/page_io.c | 12 + mm/swap.c | 4 mm/swapfile.c | 58 +++++++- 9 files changed, 496 insertions(+), 9 deletions(-) Frontswap is so named because it can be thought of as the opposite of a "backing" store for a swap device. The storage is assumed to be a synchronous concurrency-safe page-oriented pseudo-RAM device (such as Xen's Transcendent Memory, aka "tmem", or in-kernel compressed memory, aka "zmem", or other RAM-like devices) which is not directly accessible or addressable by the kernel and is of unknown and possibly time-varying size. This pseudo-RAM device links itself to frontswap by setting the frontswap_ops pointer appropriately and the functions it provides must conform to certain policies as follows: An "init" prepares the pseudo-RAM to receive frontswap pages and returns a non-negative pool id, used for all swap device numbers (aka "type"). A "put_page" will copy the page to pseudo-RAM and associate it with the type and offset associated with the page. A "get_page" will copy the page, if found, from pseudo-RAM into kernel memory, but will NOT remove the page from pseudo-RAM. A "flush_page" will remove the page from pseudo-RAM and a "flush_area" will remove ALL pages associated with the swap type (e.g., like swapoff) and notify the pseudo-RAM device to refuse further puts with that swap type. Once a page is successfully put, a matching get on the page will always succeed. So when the kernel finds itself in a situation where it needs to swap out a page, it first attempts to use frontswap. If the put returns non-zero, the data has been successfully saved to pseudo-RAM and a disk write and, if the data is later read back, a disk read are avoided. If a put returns zero, pseudo-RAM has rejected the data, and the page can be written to swap as usual. Note that if a page is put and the page already exists in pseudo-RAM (a "duplicate" put), either the put succeeds and the data is overwritten, or the put fails AND the page is flushed. This ensures stale data may never be obtained from pseudo-RAM. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to firstname.lastname@example.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"email@example.com"> firstname.lastname@example.org </a>
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