mm: balloon: update comment about isolation/migration/compaction

davidhildenbrand · torvalds · commit 4d3467e171f8 · 2019-03-05T21:07:14.000-08:00
Patch series "mm/kdump: allow to exclude pages that are logically offline" Right now, pages inflated as part of a balloon driver will be dumped by dump tools like makedumpfile. While XEN is able to check in the crash kernel whether a certain pfn is actuall backed by memory in the hypervisor (see xen_oldmem_pfn_is_ram) and optimize this case, dumps of virtio-balloon, hv-balloon and VMWare balloon inflated memory will essentially result in zero pages getting allocated by the hypervisor and the dump getting filled with this data. The allocation and reading of zero pages can directly be avoided if a dumping tool could know which pages only contain stale information not to be dumped. Also for XEN, calling into the kernel and asking the hypervisor if a pfn is backed can be avoided if the duming tool would skip such pages right from the beginning. Dumping tools have no idea whether a given page is part of a balloon driver and shall not be dumped. Esp. PG_reserved cannot be used for that purpose as all memory allocated during early boot is also PG_reserved, see discussion at [1]. So some other way of indication is required and a new page flag is frowned upon. We have PG_balloon (MAPCOUNT value), which is essentially unused now. I suggest renaming it to something more generic (PG_offline) to mark pages as logically offline. This flag can than e.g. also be used by virtio-mem in the future to mark subsections as offline. Or by other code that wants to put pages logically offline (e.g. later maybe poisoned pages that shall no longer be used). This series converts PG_balloon to PG_offline, allows dumping tools to query the value to detect such pages and marks pages in the hv-balloon and XEN balloon properly as PG_offline. Note that virtio-balloon already set pages to PG_balloon (and now PG_offline). Please note that this is also helpful for a problem we were seeing under Hyper-V: Dumping logically offline memory (pages kept fake offline while onlining a section via online_page_callback) would under some condicions result in a kernel panic when dumping them. As I don't have access to neither XEN nor Hyper-V nor VMWare installations, this was only tested with the virtio-balloon and pages were properly skipped when dumping. I'll also attach the makedumpfile patch to this series. [1] https://lkml.org/lkml/2018/7/20/566 This patch (of 8): Commit b1123ea ("mm: balloon: use general non-lru movable page feature") reworked balloon handling to make use of the general non-lru movable page feature. The big comment block in balloon_compaction.h contains quite some outdated information. Let's fix this. Link: http://lkml.kernel.org/r/20181119101616.8901-2-david@redhat.com Signed-off-by: David Hildenbrand <david@redhat.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Baoquan He <bhe@redhat.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: Christian Hansen <chansen3@cisco.com> Cc: Dave Young <dyoung@redhat.com> Cc: David Rientjes <rientjes@google.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Haiyang Zhang <haiyangz@microsoft.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Juergen Gross <jgross@suse.com> Cc: Julien Freche <jfreche@vmware.com> Cc: Kairui Song <kasong@redhat.com> Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.com> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Cc: Konstantin Khlebnikov <koct9i@gmail.com> Cc: "K. Y. Srinivasan" <kys@microsoft.com> Cc: Len Brown <len.brown@intel.com> Cc: Lianbo Jiang <lijiang@redhat.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: Miles Chen <miles.chen@mediatek.com> Cc: Nadav Amit <namit@vmware.com> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> Cc: Omar Sandoval <osandov@fb.com> Cc: Pankaj gupta <pagupta@redhat.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: Pavel Tatashin <pasha.tatashin@oracle.com> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net> Cc: Stefano Stabellini <sstabellini@kernel.org> Cc: Stephen Hemminger <sthemmin@microsoft.com> Cc: Stephen Rothwell <sfr@canb.auug.org.au> Cc: Vitaly Kuznetsov <vkuznets@redhat.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Xavier Deguillard <xdeguillard@vmware.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
@@ -4,15 +4,18 @@
  *
  * Common interface definitions for making balloon pages movable by compaction.
  *
- * Despite being perfectly possible to perform ballooned pages migration, they
- * make a special corner case to compaction scans because balloon pages are not
- * enlisted at any LRU list like the other pages we do compact / migrate.
+ * Balloon page migration makes use of the general non-lru movable page
+ * feature.
+ *
+ * page->private is used to reference the responsible balloon device.
+ * page->mapping is used in context of non-lru page migration to reference
+ * the address space operations for page isolation/migration/compaction.
  *
  * As the page isolation scanning step a compaction thread does is a lockless
  * procedure (from a page standpoint), it might bring some racy situations while
  * performing balloon page compaction. In order to sort out these racy scenarios
  * and safely perform balloon's page compaction and migration we must, always,
- * ensure following these three simple rules:
+ * ensure following these simple rules:
  *
  *   i. when updating a balloon's page ->mapping element, strictly do it under
  *      the following lock order, independently of the far superior
@@ -21,19 +24,8 @@
  *	      +--spin_lock_irq(&b_dev_info->pages_lock);
  *	            ... page->mapping updates here ...
  *
- *  ii. before isolating or dequeueing a balloon page from the balloon device
- *      pages list, the page reference counter must be raised by one and the
- *      extra refcount must be dropped when the page is enqueued back into
- *      the balloon device page list, thus a balloon page keeps its reference
- *      counter raised only while it is under our special handling;
- *
- * iii. after the lockless scan step have selected a potential balloon page for
- *      isolation, re-test the PageBalloon mark and the PagePrivate flag
- *      under the proper page lock, to ensure isolating a valid balloon page
- *      (not yet isolated, nor under release procedure)
- *
- *  iv. isolation or dequeueing procedure must clear PagePrivate flag under
- *      page lock together with removing page from balloon device page list.
+ *  ii. isolation or dequeueing procedure must remove the page from balloon
+ *      device page list under b_dev_info->pages_lock.
  *
  * The functions provided by this interface are placed to help on coping with
  * the aforementioned balloon page corner case, as well as to ensure the simple
