• Lee Schermerhorn's avatar
    mempolicy: rework mempolicy Reference Counting [yet again] · 52cd3b07
    Lee Schermerhorn authored
    
    
    After further discussion with Christoph Lameter, it has become clear that my
    earlier attempts to clean up the mempolicy reference counting were a bit of
    overkill in some areas, resulting in superflous ref/unref in what are usually
    fast paths.  In other areas, further inspection reveals that I botched the
    unref for interleave policies.
    
    A separate patch, suitable for upstream/stable trees, fixes up the known
    errors in the previous attempt to fix reference counting.
    
    This patch reworks the memory policy referencing counting and, one hopes,
    simplifies the code.  Maybe I'll get it right this time.
    
    See the update to the numa_memory_policy.txt document for a discussion of
    memory policy reference counting that motivates this patch.
    
    Summary:
    
    Lookup of mempolicy, based on (vma, address) need only add a reference for
    shared policy, and we need only unref the policy when finished for shared
    policies.  So, this patch backs out all of the unneeded extra reference
    counting added by my previous attempt.  It then unrefs only shared policies
    when we're finished with them, using the mpol_cond_put() [conditional put]
    helper function introduced by this patch.
    
    Note that shmem_swapin() calls read_swap_cache_async() with a dummy vma
    containing just the policy.  read_swap_cache_async() can call alloc_page_vma()
    multiple times, so we can't let alloc_page_vma() unref the shared policy in
    this case.  To avoid this, we make a copy of any non-null shared policy and
    remove the MPOL_F_SHARED flag from the copy.  This copy occurs before reading
    a page [or multiple pages] from swap, so the overhead should not be an issue
    here.
    
    I introduced a new static inline function "mpol_cond_copy()" to copy the
    shared policy to an on-stack policy and remove the flags that would require a
    conditional free.  The current implementation of mpol_cond_copy() assumes that
    the struct mempolicy contains no pointers to dynamically allocated structures
    that must be duplicated or reference counted during copy.
    Signed-off-by: default avatarLee Schermerhorn <lee.schermerhorn@hp.com>
    Cc: Christoph Lameter <clameter@sgi.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Mel Gorman <mel@csn.ul.ie>
    Cc: Andi Kleen <ak@suse.de>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    52cd3b07