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    SubjectRe: [PATCH]Fix: Init page count for all pages during higher order allocs
    On Thu, May 02, 2002 at 02:08:50PM +0100, Hugh Dickins wrote:
    > On Thu, 2 May 2002, Suparna Bhattacharya wrote:
    > [ discussion of PG_inuse / PG_partial / PG_large snipped ]
    > Any of those can handle that job (distinguishing non0orders),
    > but I do believe you want a further PG_ flag for crash dumps.
    > The pages allocated GFP_HIGHUSER are about as uninteresting
    > as the free pages: the cases where they're interesting (for
    > analyzing a kernel crash, as opposed to snooping on a crashed
    > customer's personal data!) are _very_ rare, but the waste of
    > space and time putting them in a crash dump is very often
    > abominable, and of course worse on larger machines.

    Well, we are working on various options to be able to dump
    pages selectively, and PG_inuse is by no means the only check.
    For example we have an option that tries to exclude non-kernel
    pages from the dump based on a simple heuristic of checking the
    PG_lru flag (actually exclude LRU pages and unreferenced pages).
    This works for vmalloc'ed pages too.

    > As someone else noted in this thread, the kernel tries to keep
    > pages in use anyway, so omitting free pages won't buy you a great
    > deal on its own. And I think it's to omit free pages that you want

    True, it is only when a system is very lightly loaded (plus not
    running for long) and has lots of memory that we'd expect
    many free pages. Maybe that not a very typical situation in a
    realistic workload, but one can envision further checks that may
    be helpful. At least in a low load situation we don't want to
    confuse free pages with kernel pages (in the example I discussed

    > to distinguish the count 0 continuations from the count 0 frees?
    > PG_highuser? PG_data? Or inverses: PG_internal? PG_dumpable?
    > I think not PG_highuser, because it's too specific to what just
    > happens to be the best, but inadequate, test I've found so far.

    I wouldn't want this kind of a flag to be specific to dump, but
    am really looking at little things that help a with generic page
    classification scheme that also addresses the needs for dump.
    We would like dump to make its decisions based on a configured
    requirement e.g depending on the dump level, and adapt or tune
    our heuristics without changing the rest of the kernel.

    The flags should just indicate the nature of the page - it's up to
    dump or any other kind of analyser to decide whether to pick it
    up or not. For different kind of situations and problems one
    might need more or less memory to be dumped, also possibly
    depending on availability of space.

    If ever we introduce anything specifically for dump, it could be
    a PG_dumped indicator to help avoid dumping already dumped pages
    in a multi-pass selection scheme, but that's something for later

    > A first guess is that pages allocated with __GFP_HIGHMEM can be
    > omitted from a dump, but that works out wrong on vmalloced space
    > and on highmem pagetables, both of which are important in a dump.
    > GFP_HIGHUSER test dumps vmalloced pages, and both Andrea's 2.4 or
    > Ingo's 2.5 highmem pagetables. But (notably in reboot after crash:
    > dump copied from swap) memory can be full of GFP_USER blockdev pages.
    > Hugh
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