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    Subjectusing highmem for atomic copy of lowmem was Re: [PATCHv2 2/5] vmscan: Kill hibernation specific reclaim logic and unify it
    Hi!

    > > >> (Disclaimer: I don't think about highmem a lot any more, and might have
    > > >> forgotten some of the details, or swsusp's algorithms might have
    > > >> changed. Rafael might need to correct some of this...)
    > > >>
    > > >> Imagine that you have a system with 1000 pages of lowmem and 5000 pages
    > > >> of highmem. Of these, 950 lowmem pages are in use and 500 highmem pages
    > > >> are in use.
    > > >>
    > > >> In order to to be able to save an image, we need to be able to do an
    > > >> atomic copy of those lowmem pages.
    > > >>
    > > >> You might think that we could just copy everything into the spare
    > > >> highmem pages, but we can't because mapping and unmapping the highmem
    > > >> pages as we copy the data will leave us with an inconsistent copy.
    > > >
    > > > This isn't the case any more for the mainline hibernate code. We use highmem
    > > > for storing image data as well as lowmem.
    > >
    > > Highmem for storing copies of lowmem pages?
    >
    > It is possible in theory, but I don't think it happens in practice given the
    > way in which the memory is freed. Still copy_data_page() takes this
    > possibility into account.

    Yes, it does, but I wonder if it can ever work...?

    copy_data_page() takes great care not to modify any memory -- like
    using handmade loop instead of memcpy() -- yet it uses kmap_atomic()
    and friends.

    If kmap_atomic()+kunmap_atomic() pair is guaranteed not to change any
    memory, thats probably safe, but...

    (Actually, plain memcpy() should be usable for copying highmem pages;
    those should not contain task_structs or anything else touched by
    memcpy()).
    Pavel

    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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