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    SubjectRe: [PATCH 02/10] mm: workingset: tell cache transitions from workingset thrashing
    On Thu, Jul 12, 2018 at 7:29 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
    > How many page->flags does this leave us with on 32-bit?
    >
    > 20 bits are always page flags
    >
    > 21 if you have an MMU
    >
    > 23 with the zone bits for DMA, Normal, HighMem, Movable
    >
    > 29 with the sparsemem section bits
    >
    > 30 if PAE is enabled
    >
    > 31 with this patch.
    >
    > So on 32-bit PAE, that leaves 1 bit for distinguishing two NUMA
    > nodes. If that's not enough, the system can switch to discontigmem and
    > re-gain the 6 or 7 sparsemem section bits.
    >
    > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

    It seems we ran out of bits on arm64 in randconfig builds:

    In file included from /git/arm-soc/include/linux/kernel.h:10,
    from /git/arm-soc/arch/arm64/mm/init.c:20:
    /git/arm-soc/arch/arm64/mm/init.c: In function 'mem_init':
    /git/arm-soc/include/linux/compiler.h:357:38: error: call to
    '__compiletime_assert_618' declared with attribute error: BUILD_BUG_ON
    failed: sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT)
    _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
    ^
    /git/arm-soc/include/linux/compiler.h:337:4: note: in definition of
    macro '__compiletime_assert'
    prefix ## suffix(); \
    ^~~~~~
    /git/arm-soc/include/linux/compiler.h:357:2: note: in expansion of
    macro '_compiletime_assert'
    _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
    ^~~~~~~~~~~~~~~~~~~
    /git/arm-soc/include/linux/build_bug.h:45:37: note: in expansion of
    macro 'compiletime_assert'
    #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
    ^~~~~~~~~~~~~~~~~~
    /git/arm-soc/include/linux/build_bug.h:69:2: note: in expansion of
    macro 'BUILD_BUG_ON_MSG'
    BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
    ^~~~~~~~~~~~~~~~
    /git/arm-soc/arch/arm64/mm/init.c:618:2: note: in expansion of macro
    'BUILD_BUG_ON'
    BUILD_BUG_ON(sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT));
    ^~~~~~~~~~~~
    /git/arm-soc/scripts/Makefile.build:317: recipe for target
    'arch/arm64/mm/init.o' failed

    Apparently this triggered

    #if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT <=
    BITS_PER_LONG - NR_PAGEFLAGS
    #define LAST_CPUPID_WIDTH LAST_CPUPID_SHIFT
    #else
    #define LAST_CPUPID_WIDTH 0
    #endif

    and in turn

    #if defined(CONFIG_NUMA_BALANCING) && LAST_CPUPID_WIDTH == 0
    #define LAST_CPUPID_NOT_IN_PAGE_FLAGS
    #endif

    and that _last_cpupid in struct page made sizeof(struct page) larger than 64.

    This is for a randconfig build, see https://pastebin.com/YuwSTah3
    for the configuration file, some of the relevant options are

    CONFIG_64BIT=y
    CONFIG_MEMCG=y
    CONFIG_SPARSEMEM=y
    CONFIG_ARM64_PA_BITS=52
    CONFIG_ARM64_64K_PAGES=y
    CONFIG_NR_CPUS=64
    CONFIG_NUMA_BALANCING=y
    # CONFIG_SPARSEMEM_VMEMMAP is not set
    CONFIG_NODES_SHIFT=2
    # CONFIG_ARCH_USES_PG_UNCACHED is not set
    CONFIG_MEMORY_FAILURE=y
    CONFIG_IDLE_PAGE_TRACKING=y

    #define MAX_NR_ZONES 3
    #define ZONES_SHIFT 2
    #define MAX_PHYSMEM_BITS 52
    #define SECTION_SIZE_BITS 30
    #define SECTIONS_WIDTH 22
    #define ZONES_WIDTH 2
    #define NODES_SHIFT 2
    #define LAST__PID_SHIFT 8
    #define NR_CPUS_BITS 6
    #define LAST_CPUPID_SHIFT 14
    #define NR_PAGEFLAGS 25

    With the extra page flag, the sum of SECTIONS_WIDTH, NODES_SHIFT, ZONES_WIDTH,
    LAST_CPUPID_SHIFT, and NR_PAGEFLAGS is now 65. Before this change, I could
    not trigger that error in randconfig builds. However, setting CONFIG_NR_CPUS or
    CONFIG_NODES_SHIFT higher than the defaults would trigger it as well (randconfig
    does not randomize those options).

    Arnd

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