![David Hildenbrand](/assets/img/avatar_default.png)
PG_balloon was introduced to implement page migration/compaction for pages inflated in virtio-balloon. Nowadays, it is only a marker that a page is part of virtio-balloon and therefore logically offline. We also want to make use of this flag in other balloon drivers - for inflated pages or when onlining a section but keeping some pages offline (e.g. used right now by XEN and Hyper-V via set_online_page_callback()). We are going to expose this flag to dump tools like makedumpfile. But instead of exposing PG_balloon, let's generalize the concept of marking pages as logically offline, so it can be reused for other purposes later on. Rename PG_balloon to PG_offline. This is an indicator that the page is logically offline, the content stale and that it should not be touched (e.g. a hypervisor would have to allocate backing storage in order for the guest to dump an unused page). We can then e.g. exclude such pages from dumps. We replace and reuse KPF_BALLOON (23), as this shouldn't really harm (and for now the semantics stay the same). In following patches, we will make use of this bit also in other balloon drivers. While at it, document PGTABLE. [akpm@linux-foundation.org: fix comment text, per David] Link: http://lkml.kernel.org/r/20181119101616.8901-3-david@redhat.com Signed-off-by: David Hildenbrand <david@redhat.com> Acked-by: Konstantin Khlebnikov <koct9i@gmail.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Pankaj gupta <pagupta@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: Christian Hansen <chansen3@cisco.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Cc: Stephen Rothwell <sfr@canb.auug.org.au> Cc: Matthew Wilcox <willy@infradead.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Pavel Tatashin <pasha.tatashin@oracle.com> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> Cc: Miles Chen <miles.chen@mediatek.com> Cc: David Rientjes <rientjes@google.com> Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.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: Dave Young <dyoung@redhat.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Haiyang Zhang <haiyangz@microsoft.com> Cc: Juergen Gross <jgross@suse.com> Cc: Julien Freche <jfreche@vmware.com> Cc: Kairui Song <kasong@redhat.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: Nadav Amit <namit@vmware.com> Cc: Omar Sandoval <osandov@fb.com> Cc: Pavel Machek <pavel@ucw.cz> 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: Vitaly Kuznetsov <vkuznets@redhat.com> Cc: Xavier Deguillard <xdeguillard@vmware.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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208 lines
7.3 KiB
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.. _pagemap:
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=============================
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Examining Process Page Tables
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=============================
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pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
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userspace programs to examine the page tables and related information by
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reading files in ``/proc``.
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There are four components to pagemap:
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* ``/proc/pid/pagemap``. This file lets a userspace process find out which
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physical frame each virtual page is mapped to. It contains one 64-bit
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value for each virtual page, containing the following data (from
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``fs/proc/task_mmu.c``, above pagemap_read):
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* Bits 0-54 page frame number (PFN) if present
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* Bits 0-4 swap type if swapped
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* Bits 5-54 swap offset if swapped
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* Bit 55 pte is soft-dirty (see
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:ref:`Documentation/admin-guide/mm/soft-dirty.rst <soft_dirty>`)
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* Bit 56 page exclusively mapped (since 4.2)
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* Bits 57-60 zero
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* Bit 61 page is file-page or shared-anon (since 3.5)
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* Bit 62 page swapped
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* Bit 63 page present
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Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs.
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In 4.0 and 4.1 opens by unprivileged fail with -EPERM. Starting from
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4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN.
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Reason: information about PFNs helps in exploiting Rowhammer vulnerability.
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If the page is not present but in swap, then the PFN contains an
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encoding of the swap file number and the page's offset into the
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swap. Unmapped pages return a null PFN. This allows determining
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precisely which pages are mapped (or in swap) and comparing mapped
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pages between processes.
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Efficient users of this interface will use ``/proc/pid/maps`` to
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determine which areas of memory are actually mapped and llseek to
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skip over unmapped regions.
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* ``/proc/kpagecount``. This file contains a 64-bit count of the number of
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times each page is mapped, indexed by PFN.
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The page-types tool in the tools/vm directory can be used to query the
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number of times a page is mapped.
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* ``/proc/kpageflags``. This file contains a 64-bit set of flags for each
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page, indexed by PFN.
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The flags are (from ``fs/proc/page.c``, above kpageflags_read):
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0. LOCKED
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1. ERROR
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2. REFERENCED
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3. UPTODATE
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4. DIRTY
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5. LRU
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6. ACTIVE
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7. SLAB
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8. WRITEBACK
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9. RECLAIM
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10. BUDDY
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11. MMAP
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12. ANON
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13. SWAPCACHE
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14. SWAPBACKED
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15. COMPOUND_HEAD
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16. COMPOUND_TAIL
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17. HUGE
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18. UNEVICTABLE
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19. HWPOISON
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20. NOPAGE
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21. KSM
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22. THP
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23. OFFLINE
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24. ZERO_PAGE
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25. IDLE
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26. PGTABLE
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* ``/proc/kpagecgroup``. This file contains a 64-bit inode number of the
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memory cgroup each page is charged to, indexed by PFN. Only available when
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CONFIG_MEMCG is set.
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Short descriptions to the page flags
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====================================
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0 - LOCKED
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page is being locked for exclusive access, e.g. by undergoing read/write IO
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7 - SLAB
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page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
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When compound page is used, SLUB/SLQB will only set this flag on the head
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page; SLOB will not flag it at all.
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10 - BUDDY
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a free memory block managed by the buddy system allocator
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The buddy system organizes free memory in blocks of various orders.
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An order N block has 2^N physically contiguous pages, with the BUDDY flag
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set for and _only_ for the first page.
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15 - COMPOUND_HEAD
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A compound page with order N consists of 2^N physically contiguous pages.
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A compound page with order 2 takes the form of "HTTT", where H donates its
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head page and T donates its tail page(s). The major consumers of compound
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pages are hugeTLB pages
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(:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`),
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the SLUB etc. memory allocators and various device drivers.
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However in this interface, only huge/giga pages are made visible
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to end users.
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16 - COMPOUND_TAIL
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A compound page tail (see description above).
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17 - HUGE
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this is an integral part of a HugeTLB page
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19 - HWPOISON
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hardware detected memory corruption on this page: don't touch the data!
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20 - NOPAGE
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no page frame exists at the requested address
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21 - KSM
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identical memory pages dynamically shared between one or more processes
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22 - THP
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contiguous pages which construct transparent hugepages
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23 - OFFLINE
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page is logically offline
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24 - ZERO_PAGE
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zero page for pfn_zero or huge_zero page
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25 - IDLE
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page has not been accessed since it was marked idle (see
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:ref:`Documentation/admin-guide/mm/idle_page_tracking.rst <idle_page_tracking>`).
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Note that this flag may be stale in case the page was accessed via
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a PTE. To make sure the flag is up-to-date one has to read
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``/sys/kernel/mm/page_idle/bitmap`` first.
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26 - PGTABLE
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page is in use as a page table
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IO related page flags
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---------------------
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1 - ERROR
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IO error occurred
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3 - UPTODATE
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page has up-to-date data
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ie. for file backed page: (in-memory data revision >= on-disk one)
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4 - DIRTY
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page has been written to, hence contains new data
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i.e. for file backed page: (in-memory data revision > on-disk one)
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8 - WRITEBACK
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page is being synced to disk
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LRU related page flags
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----------------------
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5 - LRU
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page is in one of the LRU lists
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6 - ACTIVE
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page is in the active LRU list
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18 - UNEVICTABLE
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page is in the unevictable (non-)LRU list It is somehow pinned and
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not a candidate for LRU page reclaims, e.g. ramfs pages,
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shmctl(SHM_LOCK) and mlock() memory segments
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2 - REFERENCED
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page has been referenced since last LRU list enqueue/requeue
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9 - RECLAIM
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page will be reclaimed soon after its pageout IO completed
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11 - MMAP
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a memory mapped page
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12 - ANON
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a memory mapped page that is not part of a file
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13 - SWAPCACHE
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page is mapped to swap space, i.e. has an associated swap entry
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14 - SWAPBACKED
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page is backed by swap/RAM
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The page-types tool in the tools/vm directory can be used to query the
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above flags.
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Using pagemap to do something useful
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====================================
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The general procedure for using pagemap to find out about a process' memory
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usage goes like this:
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1. Read ``/proc/pid/maps`` to determine which parts of the memory space are
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mapped to what.
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2. Select the maps you are interested in -- all of them, or a particular
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library, or the stack or the heap, etc.
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3. Open ``/proc/pid/pagemap`` and seek to the pages you would like to examine.
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4. Read a u64 for each page from pagemap.
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5. Open ``/proc/kpagecount`` and/or ``/proc/kpageflags``. For each PFN you
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just read, seek to that entry in the file, and read the data you want.
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For example, to find the "unique set size" (USS), which is the amount of
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memory that a process is using that is not shared with any other process,
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you can go through every map in the process, find the PFNs, look those up
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in kpagecount, and tally up the number of pages that are only referenced
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once.
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Other notes
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===========
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Reading from any of the files will return -EINVAL if you are not starting
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the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
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into the file), or if the size of the read is not a multiple of 8 bytes.
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Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
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always 12 at most architectures). Since Linux 3.11 their meaning changes
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after first clear of soft-dirty bits. Since Linux 4.2 they are used for
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flags unconditionally.
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