Re: [PATCH] [Request for inclusion] Filesystem in Userspace

[Andrew Morton <akpm@osdl.org>]

Linus Torvalds <torvalds@osdl.org> wrote:
>
> 
> 
> On Thu, 18 Nov 2004, Alan Cox wrote:
> > 
> > > I really do believe that user-space filesystems have problems. There's a 
> > > reason we tend to do them in kernel space. 
> > > 
> > > But limiting the outstanding writes some way may at least hide the thing.
> > 
> > Possibly dumb question. Is there a reason we can't have a prctl() that
> > flips the PF_* flags for a user space daemon in the same way as we do
> > for kernel threads that do I/O processing ?
> 
> It's more than just PF_MEMALLOC.
> 
> And PF_MEMALLOC really is to avoid _recursion_, which is the smallest
> problem. It does so by allowing the process to dip into the critical
> resources, but that only works if you know that the process is actually
> freeing pages right then and there. If you set it willy-nilly, you'll just
> run out of pages soon, and you'll be dead.

I've seen one 2.4-based project which had essentially a userspace
blockdevice driver.  Marking that special, trusted process PF_MEMALLOC did
indeed fix low-on-memory deadlocks.  Obviously it's something one does with
caution, but there are times when it makes sense.

I think there are codepaths which unconditionally turn off PF_MEMALLOC, so
they need to be tweaked to do a save/set/restore operation for it all to
work.