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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>