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2009-10-08ksm: more on default valuesHugh Dickins
Adjust the max_kernel_pages default to a quarter of totalram_pages, instead of nr_free_buffer_pages() / 4: the KSM pages themselves come from highmem, and even on a 16GB PAE machine, 4GB of KSM pages would only be pinning 32MB of lowmem with their rmap_items, so no need for the more obscure calculation (nor for its own special init function). There is no way for the user to switch KSM on if CONFIG_SYSFS is not enabled, so in that case default run to KSM_RUN_MERGE. Update KSM Documentation and Kconfig to reflect the new defaults. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-24ksm: change default values to better fit into mainline kernelIzik Eidus
Now that ksm is in mainline it is better to change the default values to better fit to most of the users. This patch change the ksm default values to be: ksm_thread_pages_to_scan = 100 (instead of 200) ksm_thread_sleep_millisecs = 20 (like before) ksm_run = KSM_RUN_STOP (instead of KSM_RUN_MERGE - meaning ksm is disabled by default) ksm_max_kernel_pages = nr_free_buffer_pages / 4 (instead of 2046) The important aspect of this patch is: it disables ksm by default, and sets the number of the kernel_pages that can be allocated to be a reasonable number. Signed-off-by: Izik Eidus <ieidus@redhat.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: unmerge is an origin of OOMsHugh Dickins
Just as the swapoff system call allocates many pages of RAM to various processes, perhaps triggering OOM, so "echo 2 >/sys/kernel/mm/ksm/run" (unmerge) is liable to allocate many pages of RAM to various processes, perhaps triggering OOM; and each is normally run from a modest admin process (swapoff or shell), easily repeated until it succeeds. So treat unmerge_and_remove_all_rmap_items() in the same way that we treat try_to_unuse(): generalize PF_SWAPOFF to PF_OOM_ORIGIN, and bracket both with that, to ask the OOM killer to kill them first, to prevent them from spawning more and more OOM kills. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: clean up obsolete referencesHugh Dickins
A few cleanups, given the munlock fix: the comment on ksm_test_exit() no longer applies, and it can be made private to ksm.c; there's no more reference to mmu_gather or tlb.h, and mmap.c doesn't need ksm.h. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: sysfs and defaultsHugh Dickins
At present KSM is just a waste of space if you don't have CONFIG_SYSFS=y to provide the /sys/kernel/mm/ksm files to tune and activate it. Make KSM depend on SYSFS? Could do, but it might be better to provide some defaults so that KSM works out-of-the-box, ready for testers to madvise MADV_MERGEABLE, even without SYSFS. Though anyone serious is likely to want to retune the numbers to their taste once they have experience; and whether these settings ever reach 2.6.32 can be discussed along the way. Save 1kB from tiny kernels by #ifdef'ing the SYSFS side of it. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: fix deadlock with munlock in exit_mmapAndrea Arcangeli
Rawhide users have reported hang at startup when cryptsetup is run: the same problem can be simply reproduced by running a program int main() { mlockall(MCL_CURRENT | MCL_FUTURE); return 0; } The problem is that exit_mmap() applies munlock_vma_pages_all() to clean up VM_LOCKED areas, and its current implementation (stupidly) tries to fault in absent pages, for example where PROT_NONE prevented them being faulted in when mlocking. Whereas the "ksm: fix oom deadlock" patch, knowing there's a race by which KSM might try to fault in pages after exit_mmap() had finally zapped the range, backs out of such faults doing nothing when its ksm_test_exit() notices mm_users 0. So revert that part of "ksm: fix oom deadlock" which moved the ksm_exit() call from before exit_mmap() to the middle of exit_mmap(); and remove those ksm_test_exit() checks from the page fault paths, so allowing the munlocking to proceed without interference. ksm_exit, if there are rmap_items still chained on this mm slot, takes mmap_sem write side: so preventing KSM from working on an mm while exit_mmap runs. And KSM will bail out as soon as it notices that mm_users is already zero, thanks to its internal ksm_test_exit checks. So that when a task is killed by OOM killer or the user, KSM will not indefinitely prevent it from running exit_mmap to release its memory. This does break a part of what "ksm: fix oom deadlock" was trying to achieve. When unmerging KSM (echo 2 >/sys/kernel/mm/ksm), and even when ksmd itself has to cancel a KSM page, it is possible that the first OOM-kill victim would be the KSM process being faulted: then its memory won't be freed until a second victim has been selected (freeing memory for the unmerging fault to complete). But the OOM killer is already liable to kill a second victim once the intended victim's p->mm goes to NULL: so there's not much point in rejecting this KSM patch before fixing that OOM behaviour. It is very much more important to allow KSM users to boot up, than to haggle over an unlikely and poorly supported OOM case. We also intend to fix munlocking to not fault pages: at which point this patch _could_ be reverted; though that would be controversial, so we hope to find a better solution. Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Acked-by: Justin M. Forbes <jforbes@redhat.com> Acked-for-now-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: Izik Eidus <ieidus@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: fix oom deadlockHugh Dickins
There's a now-obvious deadlock in KSM's out-of-memory handling: imagine ksmd or KSM_RUN_UNMERGE handling, holding ksm_thread_mutex, trying to allocate a page to break KSM in an mm which becomes the OOM victim (quite likely in the unmerge case): it's killed and goes to exit, and hangs there waiting to acquire ksm_thread_mutex. Clearly we must not require ksm_thread_mutex in __ksm_exit, simple though that made everything else: perhaps use mmap_sem somehow? And part of the answer lies in the comments on unmerge_ksm_pages: __ksm_exit should also leave all the rmap_item removal to ksmd. But there's a fundamental problem, that KSM relies upon mmap_sem to guarantee the consistency of the mm it's dealing with, yet exit_mmap tears down an mm without taking mmap_sem. And bumping mm_users won't help at all, that just ensures that the pages the OOM killer assumes are on their way to being freed will not be freed. The best answer seems to be, to move the ksm_exit callout from just before exit_mmap, to the middle of exit_mmap: after the mm's pages have been freed (if the mmu_gather is flushed), but before its page tables and vma structures have been freed; and down_write,up_write mmap_sem there to serialize with KSM's own reliance on mmap_sem. But KSM then needs to be careful, whenever it downs mmap_sem, to check that the mm is not already exiting: there's a danger of using find_vma on a layout that's being torn apart, or writing into page tables which have been freed for reuse; and even do_anonymous_page and __do_fault need to check they're not being called by break_ksm to reinstate a pte after zap_pte_range has zapped that page table. Though it might be clearer to add an exiting flag, set while holding mmap_sem in __ksm_exit, that wouldn't cover the issue of reinstating a zapped pte. All we need is to check whether mm_users is 0 - but must remember that ksmd may detect that before __ksm_exit is reached. So, ksm_test_exit(mm) added to comment such checks on mm->mm_users. __ksm_exit now has to leave clearing up the rmap_items to ksmd, that needs ksm_thread_mutex; but shift the exiting mm just after the ksm_scan cursor so that it will soon be dealt with. __ksm_enter raise mm_count to hold the mm_struct, ksmd's exit processing (exactly like its processing when it finds all VM_MERGEABLEs unmapped) mmdrop it, similar procedure for KSM_RUN_UNMERGE (which has stopped ksmd). But also give __ksm_exit a fast path: when there's no complication (no rmap_items attached to mm and it's not at the ksm_scan cursor), it can safely do all the exiting work itself. This is not just an optimization: when ksmd is not running, the raised mm_count would otherwise leak mm_structs. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: distribute remove_mm_from_listsHugh Dickins
Do some housekeeping in ksm.c, to help make the next patch easier to understand: remove the function remove_mm_from_lists, distributing its code to its callsites scan_get_next_rmap_item and __ksm_exit. That turns out to be a win in scan_get_next_rmap_item: move its remove_trailing_rmap_items and cursor advancement up, and it becomes simpler than before. __ksm_exit becomes messier, but will change again; and moving its remove_trailing_rmap_items up lets us strengthen the unstable tree item's age condition in remove_rmap_item_from_tree. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: fix endless loop on oomHugh Dickins
break_ksm has been looping endlessly ignoring VM_FAULT_OOM: that should only be a problem for ksmd when a memory control group imposes limits (normally the OOM killer will kill others with an mm until it succeeds); but in general (especially for MADV_UNMERGEABLE and KSM_RUN_UNMERGE) we do need to route the error (or kill) back to the caller (or sighandling). Test signal_pending in unmerge_ksm_pages, which could be a lengthy procedure if it has to spill into swap: returning -ERESTARTSYS so that trivial signals will restart but fatals will terminate (is that right? we do different things in different places in mm, none exactly this). unmerge_and_remove_all_rmap_items was forgetting to lock when going down the mm_list: fix that. Whether it's successful or not, reset ksm_scan cursor to head; but only if it's successful, reset seqnr (shown in full_scans) - page counts will have gone down to zero. This patch leaves a significant OOM deadlock, but it's a good step on the way, and that deadlock is fixed in a subsequent patch. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: five little cleanupsHugh Dickins
1. We don't use __break_cow entry point now: merge it into break_cow. 2. remove_all_slot_rmap_items is just a special case of remove_trailing_rmap_items: use the latter instead. 3. Extend comment on unmerge_ksm_pages and rmap_items. 4. try_to_merge_two_pages should use try_to_merge_with_ksm_page instead of duplicating its code; and so swap them around. 5. Comment on cmp_and_merge_page described last year's: update it. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: keep quiet while list emptyHugh Dickins
ksm_scan_thread already sleeps in wait_event_interruptible until setting ksm_run activates it; but if there's nothing on its list to look at, i.e. nobody has yet said madvise MADV_MERGEABLE, it's a shame to be clocking up system time and full_scans: ksmd_should_run added to check that too. And move the mutex_lock out around it: the new counts showed that when ksm_run is stopped, a little work often got done afterwards, because it had been read before taking the mutex. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: break cow once unsharedHugh Dickins
We kept agreeing not to bother about the unswappable shared KSM pages which later become unshared by others: observation suggests they're not a significant proportion. But they are disadvantageous, and it is easier to break COW to replace them by swappable pages, than offer statistics to show that they don't matter; then we can stop worrying about them. Doing this in ksm_do_scan, they don't go through cmp_and_merge_page on this pass: give them a good chance of getting into the unstable tree on the next pass, or back into the stable, by computing checksum now. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: pages_unshared and pages_volatileHugh Dickins
The pages_shared and pages_sharing counts give a good picture of how successful KSM is at sharing; but no clue to how much wasted work it's doing to get there. Add pages_unshared (count of unique pages waiting in the unstable tree, hoping to find a mate) and pages_volatile. pages_volatile is harder to define. It includes those pages changing too fast to get into the unstable tree, but also whatever other edge conditions prevent a page getting into the trees: a high value may deserve investigation. Don't try to calculate it from the various conditions: it's the total of rmap_items less those accounted for. Also show full_scans: the number of completed scans of everything registered in the mm list. The locking for all these counts is simply ksm_thread_mutex. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Acked-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: move pages_sharing updatesHugh Dickins
The pages_shared count is incremented and decremented when adding a node to and removing a node from the stable tree: easy to understand. But the pages_sharing count was hard to follow, being adjusted in various places: increment and decrement it when adding to and removing from the stable tree. And the pages_sharing variable used to include the pages_shared, then those were subtracted when shown in the pages_sharing sysfs file: now keep it as an exclusive count of leaves hanging off the stable tree nodes, throughout. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Acked-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: rename kernel_pages_allocatedHugh Dickins
We're not implementing swapping of KSM pages in its first release; but when that follows, "kernel_pages_allocated" will be a very poor name for the sysfs file showing number of nodes in the stable tree: rename that to "pages_shared" throughout. But we already have a "pages_shared", counting those page slots sharing the shared pages: first rename that to... "pages_sharing". What will become of "max_kernel_pages" when the pages shared can be swapped? I guess it will just be removed, so keep that name. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Acked-by: Izik Eidus <ieidus@redhat.com> Acked-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: change ksm nice level to be 5Izik Eidus
ksm should try not to disturb other tasks as much as possible. Signed-off-by: Izik Eidus <ieidus@redhat.com> Cc: Chris Wright <chrisw@redhat.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: Wu Fengguang <fengguang.wu@intel.com> Cc: Balbir Singh <balbir@in.ibm.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Lee Schermerhorn <lee.schermerhorn@hp.com> Cc: Avi Kivity <avi@redhat.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: change copyright messageIzik Eidus
Adding Hugh Dickins into the authors list. Signed-off-by: Izik Eidus <ieidus@redhat.com> Cc: Chris Wright <chrisw@redhat.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: Wu Fengguang <fengguang.wu@intel.com> Cc: Balbir Singh <balbir@in.ibm.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Lee Schermerhorn <lee.schermerhorn@hp.com> Cc: Avi Kivity <avi@redhat.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: Kernel SamePage MergingIzik Eidus
Ksm is code that allows merging of identical pages between one or more applications, in a way invisible to the applications that use it. Pages that are merged are marked as read-only, then COWed when any application tries to change them. Whereas fork() allows sharing anonymous pages between parent and child, ksm can share anonymous pages between unrelated processes. Ksm works by walking over the memory pages of the applications it scans, in order to find identical pages. It uses two sorted data structures, called the stable and unstable trees, to locate identical pages in an effective way. When ksm finds two identical pages, it marks them as readonly and merges them into a single page. After the pages have been marked as readonly and merged into one, Linux treats them as normal copy-on-write pages, copying to a fresh anonymous page if write access is required later. Ksm scans and merges anonymous pages only in those memory areas that have been registered with it by madvise(addr, length, MADV_MERGEABLE). The ksm scanner is controlled by sysfs files in /sys/kernel/mm/ksm/: max_kernel_pages - the maximum number of unswappable kernel pages which may be allocated by ksm (0 for unlimited). kernel_pages_allocated - how many ksm pages are currently allocated, sharing identical content between different processes (pages unswappable in this release). pages_shared - how many pages have been saved by sharing with ksm pages (kernel_pages_allocated being excluded from this count). pages_to_scan - how many pages ksm should scan before sleeping. sleep_millisecs - how many milliseconds ksm should sleep between scans. run - write 0 to disable ksm, read 0 while ksm is disabled (default), write 1 to run ksm, read 1 while ksm is running, write 2 to disable ksm and unmerge all its pages. Includes contributions by Andrea Arcangeli Chris Wright and Hugh Dickins. [hugh.dickins@tiscali.co.uk: fix rare page leak] Signed-off-by: Izik Eidus <ieidus@redhat.com> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Signed-off-by: Chris Wright <chrisw@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: Wu Fengguang <fengguang.wu@intel.com> Cc: Balbir Singh <balbir@in.ibm.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Lee Schermerhorn <lee.schermerhorn@hp.com> Cc: Avi Kivity <avi@redhat.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-09-22ksm: the mm interface to ksmHugh Dickins
This patch presents the mm interface to a dummy version of ksm.c, for better scrutiny of that interface: the real ksm.c follows later. When CONFIG_KSM is not set, madvise(2) reject MADV_MERGEABLE and MADV_UNMERGEABLE with EINVAL, since that seems more helpful than pretending that they can be serviced. But when CONFIG_KSM=y, accept them even if KSM is not currently running, and even on areas which KSM will not touch (e.g. hugetlb or shared file or special driver mappings). Like other madvices, report ENOMEM despite success if any area in the range is unmapped, and use EAGAIN to report out of memory. Define vma flag VM_MERGEABLE to identify an area on which KSM may try merging pages: leave it to ksm_madvise() to decide whether to set it. Define mm flag MMF_VM_MERGEABLE to identify an mm which might contain VM_MERGEABLE areas, to minimize callouts when forking or exiting. Based upon earlier patches by Chris Wright and Izik Eidus. Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Signed-off-by: Chris Wright <chrisw@redhat.com> Signed-off-by: Izik Eidus <ieidus@redhat.com> Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: Wu Fengguang <fengguang.wu@intel.com> Cc: Balbir Singh <balbir@in.ibm.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Lee Schermerhorn <lee.schermerhorn@hp.com> Cc: Avi Kivity <avi@redhat.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>