• Michal Hocko's avatar
    mm, memory_hotplug: do not back off draining pcp free pages from kworker context · 4b81cb2f
    Michal Hocko authored
    drain_all_pages backs off when called from a kworker context since
    commit 0ccce3b9 ("mm, page_alloc: drain per-cpu pages from workqueue
    context") because the original IPI based pcp draining has been replaced
    by a WQ based one and the check wanted to prevent from recursion and
    inter workers dependencies.  This has made some sense at the time
    because the system WQ has been used and one worker holding the lock
    could be blocked while waiting for new workers to emerge which can be a
    problem under OOM conditions.
    
    Since then commit ce612879 ("mm: move pcp and lru-pcp draining into
    single wq") has moved draining to a dedicated (mm_percpu_wq) WQ with a
    rescuer so we shouldn't depend on any other WQ activity to make a
    forward progress so calling drain_all_pages from a worker context is
    safe as long as this doesn't happen from mm_percpu_wq itself which is
    not the case because all workers are required to _not_ depend on any MM
    locks.
    
    Why is this a problem in the first place? ACPI driven memory hot-remove
    (acpi_device_hotplug) is executed from the worker context.  We end up
    calling __offline_pages to free all the pages and that requires both
    lru_add_drain_all_cpuslocked and drain_all_pages to do their job
    otherwise we can have dangling pages on pcp lists and fail the offline
    operation (__test_page_isolated_in_pageblock would see a page with 0 ref
    count but without PageBuddy set).
    
    Fix the issue by removing the worker check in drain_all_pages.
    lru_add_drain_all_cpuslocked doesn't have this restriction so it works
    as expected.
    
    Link: http://lkml.kernel.org/r/20170828093341.26341-1-mhocko@kernel.org
    Fixes: 0ccce3b9 ("mm, page_alloc: drain per-cpu pages from workqueue context")
    Signed-off-by: default avatarMichal Hocko <mhocko@suse.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: <stable@vger.kernel.org>	[4.11+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    4b81cb2f
page_alloc.c 214 KB