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    Subject[RFC][PATCH 03/13] mm/vmscan: replace implicit RECLAIM_ZONE checks with explicit checks
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    From: Dave Hansen <dave.hansen@linux.intel.com>

    RECLAIM_ZONE was assumed to be unused because it was never explicitly
    used in the kernel. However, there were a number of places where it
    was checked implicitly by checking 'node_reclaim_mode' for a zero
    value.

    These zero checks are not great because it is not obvious what a zero
    mode *means* in the code. Replace them with a helper which makes it
    more obvious: node_reclaim_enabled().

    This helper also provides a handy place to explicitly check the
    RECLAIM_ZONE bit itself. Check it explicitly there to make it more
    obvious where the bit can affect behavior.

    This should have no functional impact.

    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
    Acked-by: Christoph Lameter <cl@linux.com>
    Cc: Alex Shi <alex.shi@linux.alibaba.com>
    Cc: "Tobin C. Harding" <tobin@kernel.org>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Huang Ying <ying.huang@intel.com>
    Cc: Dan Williams <dan.j.williams@intel.com>
    Cc: Qian Cai <cai@lca.pw>
    Cc: Daniel Wagner <dwagner@suse.de>
    Cc: osalvador <osalvador@suse.de>

    --

    Note: This is not cc'd to stable. It does not fix any bugs.
    ---

    b/include/linux/swap.h | 7 +++++++
    b/mm/khugepaged.c | 2 +-
    b/mm/page_alloc.c | 2 +-
    3 files changed, 9 insertions(+), 2 deletions(-)

    diff -puN include/linux/swap.h~mm-vmscan-node_reclaim_mode_helper include/linux/swap.h
    --- a/include/linux/swap.h~mm-vmscan-node_reclaim_mode_helper 2021-01-25 16:23:08.330866712 -0800
    +++ b/include/linux/swap.h 2021-01-25 16:23:08.339866712 -0800
    @@ -12,6 +12,7 @@
    #include <linux/fs.h>
    #include <linux/atomic.h>
    #include <linux/page-flags.h>
    +#include <uapi/linux/mempolicy.h>
    #include <asm/page.h>

    struct notifier_block;
    @@ -380,6 +381,12 @@ extern int sysctl_min_slab_ratio;
    #define node_reclaim_mode 0
    #endif

    +static inline bool node_reclaim_enabled(void)
    +{
    + /* Is any node_reclaim_mode bit set? */
    + return node_reclaim_mode & (RECLAIM_ZONE|RECLAIM_WRITE|RECLAIM_UNMAP);
    +}
    +
    extern void check_move_unevictable_pages(struct pagevec *pvec);

    extern int kswapd_run(int nid);
    diff -puN mm/khugepaged.c~mm-vmscan-node_reclaim_mode_helper mm/khugepaged.c
    --- a/mm/khugepaged.c~mm-vmscan-node_reclaim_mode_helper 2021-01-25 16:23:08.332866712 -0800
    +++ b/mm/khugepaged.c 2021-01-25 16:23:08.340866712 -0800
    @@ -797,7 +797,7 @@ static bool khugepaged_scan_abort(int ni
    * If node_reclaim_mode is disabled, then no extra effort is made to
    * allocate memory locally.
    */
    - if (!node_reclaim_mode)
    + if (!node_reclaim_enabled())
    return false;

    /* If there is a count for this node already, it must be acceptable */
    diff -puN mm/page_alloc.c~mm-vmscan-node_reclaim_mode_helper mm/page_alloc.c
    --- a/mm/page_alloc.c~mm-vmscan-node_reclaim_mode_helper 2021-01-25 16:23:08.335866712 -0800
    +++ b/mm/page_alloc.c 2021-01-25 16:23:08.342866712 -0800
    @@ -3875,7 +3875,7 @@ retry:
    if (alloc_flags & ALLOC_NO_WATERMARKS)
    goto try_this_zone;

    - if (node_reclaim_mode == 0 ||
    + if (!node_reclaim_enabled() ||
    !zone_allows_reclaim(ac->preferred_zoneref->zone, zone))
    continue;

    _
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