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    SubjectRe: [PATCH 0/4] ksm - dynamic page sharing driver for linux v2
    Hi Izik,
    Is there some user documentation available? (apart from RTFS?:))
    I've compiled kernel with v2 of Your patches, loaded ksm module,
    did echo 1 > /proc/sys/kernel/mm/ksm/run, but I think it didn't do
    anything, at least no pages were collected..
    Could You advise me a bit?
    thanks a lot in advance...
    I can't wait to try it on our hosts runing 50-60 KVMs :)
    BR
    nik


    On Sat, Apr 04, 2009 at 05:35:18PM +0300, Izik Eidus wrote:
    > From v1 to v2:
    >
    > 1)Fixed security issue found by Chris Wright:
    > Ksm was checking if page is a shared page by running !PageAnon.
    > Beacuse that Ksm scan only anonymous memory, all !PageAnons
    > inside ksm data strctures are shared page, however there might
    > be a case for do_wp_page() when the VM_SHARED is used where
    > do_wp_page() would instead of copying the page into new anonymos
    > page, would reuse the page, it was fixed by adding check for the
    > dirty_bit of the virtual addresses pointing into the shared page.
    > I was not finding any VM code tha would clear the dirty bit from
    > this virtual address (due to the fact that we allocate the page
    > using page_alloc() - kernel allocated pages), ~but i still want
    > confirmation about this from the vm guys - thanks.~
    >
    > 2)Moved to sysfs to control ksm:
    > It was requested as a better way to control the ksm scanning
    > thread than ioctls.
    > the sysfs api:
    > dir: /sys/kernel/mm/ksm/
    >
    > kernel_pages_allocated - information about how many kernel pages
    > ksm have allocated, this pages are not swappable, and each page
    > like that is used by ksm to share pages with identical content
    >
    > pages_shared - how many pages were shared by ksm
    >
    > run - set to 1 when you want ksm to run, 0 when no
    >
    > max_kernel_pages - set the maximum amount of kernel pages
    > to be allocated by ksm, set 0 for unlimited.
    >
    > pages_to_scan - how many pages to scan before ksm will sleep
    >
    > sleep - how much usecs ksm will sleep.
    >
    > 3)Add sysfs paramater to control the maximum kernel pages to be by
    > ksm.
    >
    > 4)Add statistics about how much pages are really shared.
    >
    >
    > One issue still to be discussed:
    > There was a suggestion to use madvice(SHAREABLE) instead of using
    > ioctls to register memory that need to be scanned by ksm.
    > Such change is outside the area of ksm.c and would required adding
    > new madvice api, and change some parts of the vm and the kernel
    > code, so first thing to do, is realized if we really want this.
    >
    > I dont know any other open issues.
    >
    > Thanks.
    >
    > This is from the first post:
    > (The kvm part, togather with the kvm-userspace part, was post with V1
    > before about a week, whoever want to test ksm may download the
    > patch from lkml archive)
    >
    > KSM is a linux driver that allows dynamicly sharing identical memory
    > pages between one or more processes.
    >
    > Unlike tradtional page sharing that is made at the allocation of the
    > memory, ksm do it dynamicly after the memory was created.
    > Memory is periodically scanned; identical pages are identified and
    > merged.
    > The sharing is unnoticeable by the process that use this memory.
    > (the shared pages are marked as readonly, and in case of write
    > do_wp_page() take care to create new copy of the page)
    >
    > To find identical pages ksm use algorithm that is split into three
    > primery levels:
    >
    > 1) Ksm will start scan the memory and will calculate checksum for each
    > page that is registred to be scanned.
    > (In the first round of the scanning, ksm would only calculate
    > this checksum for all the pages)
    >
    > 2) Ksm will go again on the whole memory and will recalculate the
    > checmsum of the pages, pages that are found to have the same
    > checksum value, would be considered "pages that are most likely
    > wont changed"
    > Ksm will insert this pages into sorted by page content RB-tree that
    > is called "unstable tree", the reason that this tree is called
    > unstable is due to the fact that the page contents might changed
    > while they are still inside the tree, and therefore the tree would
    > become corrupted.
    > Due to this problem ksm take two more steps in addition to the
    > checksum calculation:
    > a) Ksm will throw and recreate the entire unstable tree each round
    > of memory scanning - so if we have corruption, it will be fixed
    > when we will rebuild the tree.
    > b) Ksm is using RB-tree, that its balancing is made by the node color
    > and not by the content, so even if the page get corrupted, it still
    > would take the same amount of time to search on it.
    >
    > 3) In addition to the unstable tree, ksm hold another tree that is called
    > "stable tree" - this tree is RB-tree that is sorted by the pages
    > content and all its pages are write protected, and therefore it cant get
    > corrupted.
    > Each time ksm will find two identcial pages using the unstable tree,
    > it will create new write-protected shared page, and this page will be
    > inserted into the stable tree, and would be saved there, the
    > stable tree, unlike the unstable tree, is never throwen away, so each
    > page that we find would be saved inside it.
    >
    > Taking into account the three levels that described above, the algorithm
    > work like that:
    >
    > search primary tree (sorted by entire page contents, pages write protected)
    > - if match found, merge
    > - if no match found...
    > - search secondary tree (sorted by entire page contents, pages not write
    > protected)
    > - if match found, merge
    > - remove from secondary tree and insert merged page into primary tree
    > - if no match found...
    > - checksum
    > - if checksum hasn't changed
    > - insert into secondary tree
    > - if it has, store updated checksum (note: first time this page
    > is handled it won't have a checksum, so checksum will appear
    > as "changed", so it takes two passes w/ no other matches to
    > get into secondary tree)
    > - do not insert into any tree, will see it again on next pass
    >
    > The basic idea of this algorithm, is that even if the unstable tree doesnt
    > promise to us to find two identical pages in the first round, we would
    > probably find them in the second or the third or the tenth round,
    > then after we have found this two identical pages only once, we will insert
    > them into the stable tree, and then they would be protected there forever.
    > So the all idea of the unstable tree, is just to build the stable tree and
    > then we will find the identical pages using it.
    >
    > The current implemantion can be improved alot:
    > we dont have to calculate exspensive checksum, we can just use the host
    > dirty bit.
    >
    > currently we dont support shared pages swapping (other pages that are not
    > shared can be swapped (all the pages that we didnt find to be identical
    > to other pages...).
    >
    > Walking on the tree, we keep call to get_user_pages(), we can optimized it
    > by saving the pfn, and using mmu notifiers to know when the virtual address
    > mapping was changed.
    >
    > We currently scan just programs that were registred to be used by ksm, we
    > would later want to add the abilaty to tell ksm to scan PIDS (so you can
    > scan closed binary applications as well).
    >
    > Right now ksm scanning is made by just one thread, multiple scanners
    > support might would be needed.
    >
    > This driver is very useful for KVM as in cases of runing multiple guests
    > operation system of the same type.
    > (For desktop work loads we have achived more than x2 memory overcommit
    > (more like x3))
    >
    > This driver have found users other than KVM, for example CERN,
    > Fons Rademakers:
    > "on many-core machines we run one large detector simulation program per core.
    > These simulation programs are identical but run each in their own process and
    > need about 2 - 2.5 GB RAM.
    > We typically buy machines with 2GB RAM per core and so have a problem to run
    > one of these programs per core.
    > Of the 2 - 2.5 GB about 700MB is identical data in the form of magnetic field
    > maps, detector geometry, etc.
    > Currently people have been trying to start one program, initialize the geometry
    > and field maps and then fork it N times, to have the data shared.
    > With KSM this would be done automatically by the system so it sounded extremely
    > attractive when Andrea presented it."
    >
    > I am sending another seires of patchs for kvm kernel and kvm-userspace
    > that would allow users of kvm to test ksm with it.
    > The kvm patchs would apply to Avi git tree.
    >
    >
    > Izik Eidus (4):
    > MMU_NOTIFIERS: add set_pte_at_notify()
    > add page_wrprotect(): write protecting page.
    > add replace_page(): change the page pte is pointing to.
    > add ksm kernel shared memory driver.
    >
    > include/linux/ksm.h | 48 ++
    > include/linux/miscdevice.h | 1 +
    > include/linux/mm.h | 5 +
    > include/linux/mmu_notifier.h | 34 +
    > include/linux/rmap.h | 11 +
    > mm/Kconfig | 6 +
    > mm/Makefile | 1 +
    > mm/ksm.c | 1668 ++++++++++++++++++++++++++++++++++++++++++
    > mm/memory.c | 90 +++-
    > mm/mmu_notifier.c | 20 +
    > mm/rmap.c | 139 ++++
    > 11 files changed, 2021 insertions(+), 2 deletions(-)
    > create mode 100644 include/linux/ksm.h
    > create mode 100644 mm/ksm.c
    >
    > --
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    >

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