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    SubjectRe: [PATCH] [Request for inclusion] Filesystem in Userspace
    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.
    -
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