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    SubjectRe: [PATCH 10/10] mm: per device dirty threshold
    On Mon, 23 Apr 2007 08:29:59 +0200 Miklos Szeredi <miklos@szeredi.hu> wrote:

    > > > What about swapout? That can increase the number of writeback pages,
    > > > without decreasing the number of dirty pages, no?
    > >
    > > Could we not solve that by enabling cap_account_writeback on
    > > swapper_space, and thereby account swap writeback pages. Then the VM
    > > knows it has outstanding IO and need not panic.
    >
    > Hmm, I'm not sure that would be right, because then those writeback
    > pages would be accounted twice: once for swapper_space, and once for
    > the real device.
    >
    > So there's a condition, when lots of anonymous pages are turned into
    > swap-cache writeback pages, and we should somehow throttle this, because
    >
    > >>> This means that all memory is pinned and unreclaimable and the VM gets
    > >>> upset and goes oom.
    >
    > although, it's not quite clear in my mind, how the VM gets upset about
    > this.

    I've been scratching my head on and off for a couple of days over this.

    We've traditionally had reclaim problems when there's a huge amount of
    dirty MAP_SHARED data, which the VM didn't know was dirty. It's the old
    "map a file which is the same size as physical memory and write to it all"
    stresstest.

    But we do not have such problems with anonymous memory, and I'm darned if I
    can remember why :(

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