• Minchan Kim's avatar
    mm: migrate: support non-lru movable page migration · bda807d4
    Minchan Kim authored
    We have allowed migration for only LRU pages until now and it was enough
    to make high-order pages.  But recently, embedded system(e.g., webOS,
    android) uses lots of non-movable pages(e.g., zram, GPU memory) so we
    have seen several reports about troubles of small high-order allocation.
    For fixing the problem, there were several efforts (e,g,.  enhance
    compaction algorithm, SLUB fallback to 0-order page, reserved memory,
    vmalloc and so on) but if there are lots of non-movable pages in system,
    their solutions are void in the long run.
    
    So, this patch is to support facility to change non-movable pages with
    movable.  For the feature, this patch introduces functions related to
    migration to address_space_operations as well as some page flags.
    
    If a driver want to make own pages movable, it should define three
    functions which are function pointers of struct
    address_space_operations.
    
    1. bool (*isolate_page) (struct page *page, isolate_mode_t mode);
    
    What VM expects on isolate_page function of driver is to return *true*
    if driver isolates page successfully.  On returing true, VM marks the
    page as PG_isolated so concurrent isolation in several CPUs skip the
    page for isolation.  If a driver cannot isolate the page, it should
    return *false*.
    
    Once page is successfully isolated, VM uses page.lru fields so driver
    shouldn't expect to preserve values in that fields.
    
    2. int (*migratepage) (struct address_space *mapping,
    		struct page *newpage, struct page *oldpage, enum migrate_mode);
    
    After isolation, VM calls migratepage of driver with isolated page.  The
    function of migratepage is to move content of the old page to new page
    and set up fields of struct page newpage.  Keep in mind that you should
    indicate to the VM the oldpage is no longer movable via
    __ClearPageMovable() under page_lock if you migrated the oldpage
    successfully and returns 0.  If driver cannot migrate the page at the
    moment, driver can return -EAGAIN.  On -EAGAIN, VM will retry page
    migration in a short time because VM interprets -EAGAIN as "temporal
    migration failure".  On returning any error except -EAGAIN, VM will give
    up the page migration without retrying in this time.
    
    Driver shouldn't touch page.lru field VM using in the functions.
    
    3. void (*putback_page)(struct page *);
    
    If migration fails on isolated page, VM should return the isolated page
    to the driver so VM calls driver's putback_page with migration failed
    page.  In this function, driver should put the isolated page back to the
    own data structure.
    
    4. non-lru movable page flags
    
    There are two page flags for supporting non-lru movable page.
    
    * PG_movable
    
    Driver should use the below function to make page movable under
    page_lock.
    
    	void __SetPageMovable(struct page *page, struct address_space *mapping)
    
    It needs argument of address_space for registering migration family
    functions which will be called by VM.  Exactly speaking, PG_movable is
    not a real flag of struct page.  Rather than, VM reuses page->mapping's
    lower bits to represent it.
    
    	#define PAGE_MAPPING_MOVABLE 0x2
    	page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
    
    so driver shouldn't access page->mapping directly.  Instead, driver
    should use page_mapping which mask off the low two bits of page->mapping
    so it can get right struct address_space.
    
    For testing of non-lru movable page, VM supports __PageMovable function.
    However, it doesn't guarantee to identify non-lru movable page because
    page->mapping field is unified with other variables in struct page.  As
    well, if driver releases the page after isolation by VM, page->mapping
    doesn't have stable value although it has PAGE_MAPPING_MOVABLE (Look at
    __ClearPageMovable).  But __PageMovable is cheap to catch whether page
    is LRU or non-lru movable once the page has been isolated.  Because LRU
    pages never can have PAGE_MAPPING_MOVABLE in page->mapping.  It is also
    good for just peeking to test non-lru movable pages before more
    expensive checking with lock_page in pfn scanning to select victim.
    
    For guaranteeing non-lru movable page, VM provides PageMovable function.
    Unlike __PageMovable, PageMovable functions validates page->mapping and
    mapping->a_ops->isolate_page under lock_page.  The lock_page prevents
    sudden destroying of page->mapping.
    
    Driver using __SetPageMovable should clear the flag via
    __ClearMovablePage under page_lock before the releasing the page.
    
    * PG_isolated
    
    To prevent concurrent isolation among several CPUs, VM marks isolated
    page as PG_isolated under lock_page.  So if a CPU encounters PG_isolated
    non-lru movable page, it can skip it.  Driver doesn't need to manipulate
    the flag because VM will set/clear it automatically.  Keep in mind that
    if driver sees PG_isolated page, it means the page have been isolated by
    VM so it shouldn't touch page.lru field.  PG_isolated is alias with
    PG_reclaim flag so driver shouldn't use the flag for own purpose.
    
    [opensource.ganesh@gmail.com: mm/compaction: remove local variable is_lru]
      Link: http://lkml.kernel.org/r/20160618014841.GA7422@leo-test
    Link: http://lkml.kernel.org/r/1464736881-24886-3-git-send-email-minchan@kernel.orgSigned-off-by: default avatarGioh Kim <gi-oh.kim@profitbricks.com>
    Signed-off-by: default avatarMinchan Kim <minchan@kernel.org>
    Signed-off-by: default avatarGanesh Mahendran <opensource.ganesh@gmail.com>
    Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Rafael Aquini <aquini@redhat.com>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: John Einar Reitan <john.reitan@foss.arm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    bda807d4
ksm.h 2.83 KB