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    SubjectRe: [patch] VM accounting change
    Terence Ripperda <> wrote:
    > On Thu, Nov 11, 2004 at 03:07:10PM -0800, wrote:
    > > VM_LOCKED|VM_IO doesn't seem to be a sane combination. VM_LOCKED means
    > > "don't page it out" and VM_IO means "an IO region". The kernel never even
    > > attempts to page out IO regions because they don't have reverse mappings.
    > > Heck, they don't even have pageframes.
    > >
    > > How about you drop the VM_LOCKED?
    > sounds good, I can do that.
    > on a related note, there are a couple of flags that I'm not 100% clear
    > on the difference between, mainly:
    > PG_locked
    > PG_reserved
    > everything I've seen in the past has suggested that drivers set the
    > PG_reserved flag for memory allocations intended to be locked down in
    > memory for extensive dma (the bttv driver had always been pointed to
    > as an example of that).
    > I'm not clear how that differs from PG_locked and VM_LOCKED. is
    > PG_reserved still the suggested way to properly lock memory down, or
    > is there a more generally accepted method?

    VM_LOCKED means that someone did mlock() and the VMA isn't eligible for

    PG_locked is very different: it provides the caller with exclusive access
    the page while its actual contents are being changed. It's also used as a
    synchronisation point for adding to and removing from pagecache. It's
    pretty much a pagecache concept rather than an MM concept.

    PG_reserved does mean that the page is "special" and the VM should just
    leave the thing alone - some device driver owns the page and knows how to
    manage it.

    VM_RESERVED is a bit of a mystery, really and we've had some trouble over
    the semantics of this vs PG_reserved. Presumably it's supposed to be like
    PG_reserved, only for whole mmap regions. It may not work properly because
    it gets damn little testing.

    We really should have gone through and rationalised, consolidated and
    documented the PageReserved/VM_RESERVED code in the 2.5 cycle but it didn't
    happen. The most noxious part is all the testing of PG_reserved in the
    core kernel page refcounting logic.

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