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    SubjectRe: Maybe a VM bug in 2.4.18-18 from RH 8.0?
    William Lee Irwin III wrote:
    > Yes, it's necessary; no, I've never directly encountered the issue it
    > fixes. Sorry about the miscommunication there.

    The google thing.

    The basic problem is in allowing allocations which _could_ use
    highmem to use the normal zone as anon memory or pagecache.

    Because the app could mlock that memory. So for a simple

    - mem=2G
    - read a 1.2G file
    - malloc 800M, now mlock it.

    Those 800M will be in ZONE_NORMAL, simply because that was where the
    free memory was. And you're dead, even though you've only mlocked
    800M. The same thing happens if you have lots of anon memory in the
    normal zone and there is no swapspace available.

    Linus's approach was to raise the ZONE_NORMAL pages_min limit for
    allocations which _could_ use highmem. So a GFP_HIGHUSER allocation
    has a pages_min limit of (say) 4M when considering the normal zone,
    but a GFP_KERNEL allocation has a limit of 2M.

    Andrea's patch does the same thing, via a separate table. He has
    set the threshold much higher (100M on a 4G box). AFAICT, the
    algorithms are identical - I was planning on just adding a multiplier
    to set Linus's ratio - it is currently hardwired to "1". Search for
    "mysterious" in mm/page_alloc.c ;)

    It's not clear to me why -aa defaults to 100 megs when the problem
    only occurs with no swap or when the app is using mlock. The default
    multiplier (of variable local_min) should be zero. Swapless machines
    or heavy mlock users can crank it up.

    But mlocking 700M on a 4G box would kill it as well. The google
    application, IIRC, mlocks 1G on a 2G machine. Daniel put them
    onto the 2G+2G split and all was well.

    Anyway, thanks. I'll take another look at Andrea's implementation.

    Now, regarding mlock(mmap(open(/dev/hda1))) ;)
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