Re: [PATCH 2/12] mm: let swap use exceptional entries

[Hugh Dickins]

On Sat, 18 Jun 2011, Andrew Morton wrote:
> On Tue, 14 Jun 2011 03:43:47 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> 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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