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    SubjectRe: [[PATCH]] mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged
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    On 9/17/2020 5:12 AM, Michal Hocko wrote:
    > On Wed 16-09-20 11:28:40, Vijay Balakrishna wrote:
    > [...]
    >> OOM splat below. I see we had kmem leak detection turned on here. We
    >> haven't run stress with kmem leak detection since uncovereing low
    >> min_free_kbytes. During investigation we wanted to make sure there is no
    >> kmem leaks, we didn't find significant leaks detected.
    >>
    >> [330319.766059] systemd invoked oom-killer:
    >> gfp_mask=0x40cc0(GFP_KERNEL|__GFP_COMP), order=1, oom_score_adj=0
    >
    > [...]
    >> [330319.861064] Mem-Info:
    >> [330319.863519] active_anon:60744 inactive_anon:109226 isolated_anon:0
    >> active_file:6418 inactive_file:3869 isolated_file:2
    >> unevictable:0 dirty:8 writeback:1 unstable:0
    >> slab_reclaimable:34660 slab_unreclaimable:795718
    >> mapped:1256 shmem:165765 pagetables:689 bounce:0
    >> free:340962 free_pcp:4672 free_cma:0
    >
    > The memory consumption is predominantely in slab (unreclaimable). Only
    > ~8% of the memory is on LRUs (anonymous + file). Slab (both reclaimable
    > and unreclaimable) is ~40%. So there is still a lot of memory
    > unaccounted (direct users of the page allocator). This would partially
    > explain why the oom killer is not able to make progress and eventually
    > panics because it is the kernel which is blowing the memory consumption.
    >
    > There is still ~1G free memory but the problem is that this is a
    > GFP_KERNEL request which is not allowed to consume Movable memory.
    > Zone normal is depleted and therefore it cannot satisfy this request
    > even when there are some order-1 pages available.
    >
    >> [330319.928124] Node 0 Normal free:12652kB min:14344kB low:19092kB=20
    >> high:23840kB active_anon:55340kB inactive_anon:60276kB active_file:60kB
    >> inactive_file:128kB unevictable:0kB writepending:4kB present:6220656kB
    >> managed:4750196kB mlocked:0kB kernel_stack:9568kB pagetables:2756kB
    >> bounce:0kB free_pcp:10056kB local_pcp:1376kB free_cma:0kB
    > [...]
    >> [330319.996879] Node 0 Normal: 3138*4kB (UME) 38*8kB (UM) 0*16kB 0*32kB
    >> 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 12856kB
    >
    > I do not see the state of swap in the oom splat so I assume you have
    > swap disabled. If that is the case then the memory reclaim cannot really
    > do much for this request. There is almost no page cache to reclaim.

    No swap configured in our system.
    >
    > That being said I do not see how a increased min_free_kbytes could help
    > for this particular OOM situation. If there is really any relation it is
    > more of a unintended side effect.

    I haven't had a chance to rerun stress with kmem leak detection to know
    if we still see OOM kills after min_free_kbytes restore.
    >
    > [...]
    >>>> Extreme values can damage your system. Setting min_free_kbytes to an
    >>>> extremely low value prevents the system from reclaiming memory, which can
    >>>> result in system hangs and OOM-killing processes. However, setting
    >>>> min_free_kbytes too high (for example, to 5–10% of total system memory)
    >>>> causes the system to enter an out-of-memory state immediately, resulting in
    >>>> the system spending too much time reclaiming memory.
    >>>
    >>> The auto tuned value should never reach such a low value to cause
    >>> problems.
    >>
    >> The auto tuned value is incorrect post hotplug memory operation, in our use
    >> case memoy hot add occurs very early during boot.
    >
    > Define incorrect. What are the actual values? Have you tried to increase
    > the value manually after the hotplug?

    In our case SoC with 8GB memory, system tuned min_free_kbytes
    - first to 22528
    - we perform memory hot add very early in boot
    - now min_free_kbytes is 8703

    Before looking at code, first I manually restored min_free_kbytes soon
    after boot, reran stress and didn't notice symptoms I mentioned in
    change log.

    Thanks,
    Vijay

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