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    SubjectRe: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM
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    >> There was a report that this results in undesired side effects when
    >> inflating the balloon to shrink the page cache. [1]
    >> "When inflating the balloon against page cache (i.e. no free memory
    >> remains) vmscan.c will both shrink page cache, but also invoke the
    >> shrinkers -- including the balloon's shrinker. So the balloon
    >> driver allocates memory which requires reclaim, vmscan gets this
    >> memory by shrinking the balloon, and then the driver adds the
    >> memory back to the balloon. Basically a busy no-op."
    >>
    >> The name "deflate on OOM" makes it pretty clear when deflation should
    >> happen - after other approaches to reclaim memory failed, not while
    >> reclaiming. This allows to minimize the footprint of a guest - memory
    >> will only be taken out of the balloon when really needed.
    >>
    >> Especially, a drop_slab() will result in the whole balloon getting
    >> deflated - undesired.
    >
    > Could you explain why some more? drop_caches shouldn't be really used in
    > any production workloads and if somebody really wants all the cache to
    > be dropped then why is balloon any different?
    >

    Deflation should happen when the guest is out of memory, not when
    somebody thinks it's time to reclaim some memory. That's what the
    feature promised from the beginning: Only give the guest more memory in
    case it *really* needs more memory.

    Deflate on oom, not deflate on reclaim/memory pressure. (that's what the
    report was all about)

    A priority for shrinkers might be a step into the right direction.

    --
    Thanks,

    David / dhildenb

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