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    SubjectRe: [PATCH] mm: limit lowmem_reserve
    Con Kolivas wrote:
    > I hate to resuscitate this old thread, sorry but I'm still not sure we
    > resolved it and I want to make sure this issue isn't here as I see it.

    OK, reclaim is slightly different.

    > On Saturday 08 April 2006 11:25, Nick Piggin wrote:
    >>Con Kolivas wrote:
    >>>Ok. I think I presented enough information for why I thought
    >>>zone_watermark_ok would fail (for ZONE_DMA). With 16MB ZONE_DMA and a
    >>>vmsplit of 3GB we have a lowmem_reserve of 12MB. It's pretty hard to keep
    >>>that much ZONE_DMA free, I don't think I've ever seen that much free on
    >>>my ZONE_DMA on an ordinary desktop without any particular ZONE_DMA users.
    >>>Changing the tunable can make the lowmem_reserve larger than ZONE_DMA is
    >>>on any vmsplit too as far as I understand the ratio.
    >>Umm, for ZONE_DMA allocations, ZONE_DMA isn't a lower zone. So that
    >>12MB protection should never come into it (unless it is buggy?).
    > An i386 pc with a 3GB split will have approx
    > 4000 pages ZONE_DMA
    > and lowmem reserve will set lowmem reserve to approx
    > 0 0 3000 3000
    > So if we call zone_watermark_ok with zone of ZONE_DMA and a classzone_idx of a
    > ZONE_NORMAL we will fail a zone_watermark_ok test almost always since it's
    > almost impossible to have 3000 free ZONE_DMA pages. I believe it can happen
    > like this:
    > In balance_pgdat (vmscan.c:1116) if we end up with end_zone being a
    > ZONE_NORMAL zone, then during the scan below we (vmscan.c:1137) iterate over
    > all zones from 0 to end_zone and (vmscan.c:1147) we end up calling
    > if (!zone_watermark_ok(zone, order, zone->pages_high, end_zone, 0))
    > which would now call zone_watermark_ok with zone being a ZONE_DMA, and
    > end_zone being the idx of a ZONE_NORMAL.
    > So in summary if I'm not mistaken (and I'm good at being mistaken), if we
    > balance pgdat and find that ZONE_NORMAL or higher needs scanning, we'll end
    > up trying to flush the crap out of ZONE_DMA.

    If we're under memory pressure, kswapd will try to free up any candidate
    zone, yes.

    > On my test case this indeed happens and my ZONE_DMA never goes below 3000
    > pages free. If I lower the reserve even further my pages free gets stuck at
    > 3208 and can't free any more, and doesn't ever drop below that either.
    > Here is the patch I was proposing

    What problem does that fix though?

    SUSE Labs, Novell Inc.
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