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    SubjectRe: [PATCH 2/12] mm: let swap use exceptional entries
    On Sat, 18 Jun 2011, Andrew Morton wrote:
    > On Tue, 14 Jun 2011 03:43:47 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:
    >
    > > In an i386 kernel this limits its information (type and page offset)
    > > to 30 bits: given 32 "types" of swapfile and 4kB pagesize, that's
    > > a maximum swapfile size of 128GB. Which is less than the 512GB we
    > > previously allowed with X86_PAE (where the swap entry can occupy the
    > > entire upper 32 bits of a pte_t), but not a new limitation on 32-bit
    > > without PAE; and there's not a new limitation on 64-bit (where swap
    > > filesize is already limited to 16TB by a 32-bit page offset).
    >
    > hm.
    >
    > > Thirty
    > > areas of 128GB is probably still enough swap for a 64GB 32-bit machine.
    >
    > What if it was only one area? 128GB is close enough to 64GB (or, more
    > realistically, 32GB) to be significant. For the people out there who
    > are using a single 200GB swap partition and actually needed that much,
    > what happens? swapon fails?

    No, it doesn't fail: it just trims back the amount of swap that is used
    (and counted) to the maximum that the running kernel supports (just like
    when you switch between 64bit and 32bit-PAE and 32bit-nonPAE kernels
    using the same large swap device, the 64bit being able to access more
    of it than the 32bit-PAE kernel, and that more than the 32bit-nonPAE).

    I'd grown to think that the users of large amounts of RAM may like to
    have a little swap for leeway, but live in dread of the slow death that a
    large amount of swap can result in. Maybe that's just one class of user.

    I'd worry more about this if it were a new limitation for 64bit; but it's
    just a lower limitation for the 32bit-PAE case. If it actually proves
    to be an issue (and we abandon our usual mantra to go to 64bit), then I
    don't think having 32 distinct areas is sacrosanct: we can (configurably
    or tunably) lower the number of areas and increase their size; but I
    doubt we shall need to bother.

    ARM is getting LPAE? Then I guess this is a good moment to enforce
    the new limit.

    Hugh


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