• Johannes Weiner's avatar
    mm: memcontrol: rewrite uncharge API · 0a31bc97
    Johannes Weiner authored
    The memcg uncharging code that is involved towards the end of a page's
    lifetime - truncation, reclaim, swapout, migration - is impressively
    complicated and fragile.
    
    Because anonymous and file pages were always charged before they had their
    page->mapping established, uncharges had to happen when the page type
    could still be known from the context; as in unmap for anonymous, page
    cache removal for file and shmem pages, and swap cache truncation for swap
    pages.  However, these operations happen well before the page is actually
    freed, and so a lot of synchronization is necessary:
    
    - Charging, uncharging, page migration, and charge migration all need
      to take a per-page bit spinlock as they could race with uncharging.
    
    - Swap cache truncation happens during both swap-in and swap-out, and
      possibly repeatedly before the page is actually freed.  This means
      that the memcg swapout code is called from many contexts that make
      no sense and it has to figure out the direction from page state to
      make sure memory and memory+swap are always correctly charged.
    
    - On page migration, the old page might be unmapped but then reused,
      so memcg code has to prevent untimely uncharging in that case.
      Because this code - which should be a simple charge transfer - is so
      special-cased, it is not reusable for replace_page_cache().
    
    But now that charged pages always have a page->mapping, introduce
    mem_cgroup_uncharge(), which is called after the final put_page(), when we
    know for sure that nobody is looking at the page anymore.
    
    For page migration, introduce mem_cgroup_migrate(), which is called after
    the migration is successful and the new page is fully rmapped.  Because
    the old page is no longer uncharged after migration, prevent double
    charges by decoupling the page's memcg association (PCG_USED and
    pc->mem_cgroup) from the page holding an actual charge.  The new bits
    PCG_MEM and PCG_MEMSW represent the respective charges and are transferred
    to the new page during migration.
    
    mem_cgroup_migrate() is suitable for replace_page_cache() as well,
    which gets rid of mem_cgroup_replace_page_cache().  However, care
    needs to be taken because both the source and the target page can
    already be charged and on the LRU when fuse is splicing: grab the page
    lock on the charge moving side to prevent changing pc->mem_cgroup of a
    page under migration.  Also, the lruvecs of both pages change as we
    uncharge the old and charge the new during migration, and putback may
    race with us, so grab the lru lock and isolate the pages iff on LRU to
    prevent races and ensure the pages are on the right lruvec afterward.
    
    Swap accounting is massively simplified: because the page is no longer
    uncharged as early as swap cache deletion, a new mem_cgroup_swapout() can
    transfer the page's memory+swap charge (PCG_MEMSW) to the swap entry
    before the final put_page() in page reclaim.
    
    Finally, page_cgroup changes are now protected by whatever protection the
    page itself offers: anonymous pages are charged under the page table lock,
    whereas page cache insertions, swapin, and migration hold the page lock.
    Uncharging happens under full exclusion with no outstanding references.
    Charging and uncharging also ensure that the page is off-LRU, which
    serializes against charge migration.  Remove the very costly page_cgroup
    lock and set pc->flags non-atomically.
    
    [mhocko@suse.cz: mem_cgroup_charge_statistics needs preempt_disable]
    [vdavydov@parallels.com: fix flags definition]
    Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Vladimir Davydov <vdavydov@parallels.com>
    Tested-by: default avatarJet Chen <jet.chen@intel.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
    Tested-by: default avatarFelipe Balbi <balbi@ti.com>
    Signed-off-by: default avatarVladimir Davydov <vdavydov@parallels.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    0a31bc97
swap.h 18.2 KB