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SubjectMemory Mapped Filesystem
Hello,

I converted yesterday an Alpha station to ext3 (and it works great).
However, I kept /tmp as ext2 since the README distributed with ext3 says
that it would be more efficient this way. This is somehow inelegant,
since this means that there is still one fsck (which can fail).

I could do an automatic mke2fs on the device, at boot time, since this
is /tmp. However, with automatic numbering of partitions, I feel a bit
unsafe with a mke2fs in a boot script.

So I remembered that, under FreeBSD, I use mfs for /tmp: a filesystem
mapped in memory. This is not quite a ramdisk since:
-- it can be swapped out
-- it frees unsued pages when a file is removed
Somehow, this is like sharing space for /tmp and swap. I find this
scheme elegant and it would fit my needs.

Therefore, I intend to implement such a filesystem. For this, I need
some help; specifically, I ask myself the following questions:
-- does such a filesystem already exists ?
-- would it be easier to modify the ramdisk to be able to be
swapped out, and stubs in one filesystem to unmap unused pages ?
-- how do I allocate memory from within the kernel, such that this
memory might be swapped out ? I can do a do_mmap(), but, from
my (rather low) understanding of the memory management in Linux,
this mapping would be specific to one process. I could run
a kernel thread (well, at least I believe I could) but I would
run into the "Minix-single-threaded-filesystem".
-- what happens if I create a file on such a filesystem and try to
swapon it ?
-- does all this make sense ? Is there a better solution ?

Thanks for any information,

--Thomas Pornin

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