On Thu, 27 Sep 2001, Pavel Machek wrote:

Hi!

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So my suggestion was to look at getting anonymous pages backed by what
amounts to a shared memory segment.  In that vein.  By using an extent
based data structure we can get the cost down under the current 8 bits
per page that we have for the swap counts, and make allocating swap
pages faster.  And we want to cluster related swap pages anyway so
an extent based system is a natural fit.

Much of this goes away if you get rid of both the swap and anonymous page
special cases. Back anonymous pages with the "whoops everything I write here
vanishes mysteriously" file system and swap with a swapfs

What exactly is anonymous memory? I thought it is what you do when you
want to malloc(), but you want to back that up by swap, not /dev/null.

Anonymous memory is memory which is not backed by a filesystem or a
device. eg: malloc()ed memory, shmem, mmap(MAP_PRIVATE) on a file (which
will create anonymous memory as soon as the program which did the mmap
writes to the mapped memory (COW)), etc.

So... how can alan propose to back anonymous memory with /dev/null?

I guess he means anonymous memory backed up by /dev/null means anonymous
memory backep up by nothing.

[see above] It should be backed by swap, no?

Not necessarily. As soon as we need to swapout anon memory, we have to
back it up by swap. (mm/vmscan.c:try_to_swap_out() job)

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