• Shakeel Butt's avatar
    mm, mlock, vmscan: no more skipping pagevecs · 9c4e6b1a
    Shakeel Butt authored
    When a thread mlocks an address space backed either by file pages which
    are currently not present in memory or swapped out anon pages (not in
    swapcache), a new page is allocated and added to the local pagevec
    (lru_add_pvec), I/O is triggered and the thread then sleeps on the page.
    On I/O completion, the thread can wake on a different CPU, the mlock
    syscall will then sets the PageMlocked() bit of the page but will not be
    able to put that page in unevictable LRU as the page is on the pagevec
    of a different CPU.  Even on drain, that page will go to evictable LRU
    because the PageMlocked() bit is not checked on pagevec drain.
    
    The page will eventually go to right LRU on reclaim but the LRU stats
    will remain skewed for a long time.
    
    This patch puts all the pages, even unevictable, to the pagevecs and on
    the drain, the pages will be added on their LRUs correctly by checking
    their evictability.  This resolves the mlocked pages on pagevec of other
    CPUs issue because when those pagevecs will be drained, the mlocked file
    pages will go to unevictable LRU.  Also this makes the race with munlock
    easier to resolve because the pagevec drains happen in LRU lock.
    
    However there is still one place which makes a page evictable and does
    PageLRU check on that page without LRU lock and needs special attention.
    TestClearPageMlocked() and isolate_lru_page() in clear_page_mlock().
    
    	#0: __pagevec_lru_add_fn	#1: clear_page_mlock
    
    	SetPageLRU()			if (!TestClearPageMlocked())
    					  return
    	smp_mb() // <--required
    					// inside does PageLRU
    	if (!PageMlocked())		if (isolate_lru_page())
    	  move to evictable LRU		  putback_lru_page()
    	else
    	  move to unevictable LRU
    
    In '#1', TestClearPageMlocked() provides full memory barrier semantics
    and thus the PageLRU check (inside isolate_lru_page) can not be
    reordered before it.
    
    In '#0', without explicit memory barrier, the PageMlocked() check can be
    reordered before SetPageLRU().  If that happens, '#0' can put a page in
    unevictable LRU and '#1' might have just cleared the Mlocked bit of that
    page but fails to isolate as PageLRU fails as '#0' still hasn't set
    PageLRU bit of that page.  That page will be stranded on the unevictable
    LRU.
    
    There is one (good) side effect though.  Without this patch, the pages
    allocated for System V shared memory segment are added to evictable LRUs
    even after shmctl(SHM_LOCK) on that segment.  This patch will correctly
    put such pages to unevictable LRU.
    
    Link: http://lkml.kernel.org/r/20171121211241.18877-1-shakeelb@google.comSigned-off-by: 's avatarShakeel Butt <shakeelb@google.com>
    Acked-by: 's avatarVlastimil Babka <vbabka@suse.cz>
    Cc: Jérôme Glisse <jglisse@redhat.com>
    Cc: Huang Ying <ying.huang@intel.com>
    Cc: Tim Chen <tim.c.chen@linux.intel.com>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Greg Thelen <gthelen@google.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Balbir Singh <bsingharora@gmail.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Shaohua Li <shli@fb.com>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Nicholas Piggin <npiggin@gmail.com>
    Cc: Dan Williams <dan.j.williams@intel.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    9c4e6b1a
mlock.c 22.6 KB