32GB limit per swap device?

[Alan Cox]

On 08/22/2011 21:36, Matthew Dillon wrote:
>      The limitation was ONLY due to a *minor* 32-bit integer overflow in one
>      or two *intermediate* calculations in the radix tree code, which I
>      long ago fixed in DragonFly.
>
>      Just find the changes in the DFly codebase and determine if they need
>      to be applied.
>
>      The swap space radix code (which I wrote long ago) is in page-sized
>      blocks, so you actually probably want to keep using a 32-bit integer for
>      the block number there to keep the physical memory reservation required
>      for the radix tree low.  If you just pop the base block id up to 64 bits
>      without adjusting the radix code to overlay a 64 bit bitmap on it you
>      waste a lot of physical memory for the same amount of swap reservation.

Unfortunately, in FreeBSD, when daddr_t was increased to 64 bits a few 
years ago, the bitmap size was not increased.  So, we have been wasting 
about half the space used by the blist structure for some time now.  I 
expect that we'll fix this after the current code freeze ends.

However, as Alexander observed, the primary reason for the 32GB limit on 
the size of a swap partition was artificial.  The limit was being 
implemented in the wrong place.  It was being performed before the 
conversion from 512 byte blocks to 4KB pages.  Since a blist is managing 
swap space at the granularity of pages and the limit is imposed by the 
blist code, the check should come after the conversion.  Just by moving 
the check to its proper place, the effective limit can be increased from 
32GB to 256GB.  Kostik committed such a change earlier today.

Alan