Re: [PATCH] __free_pages_ok oops

[Linus Torvalds]

On Thu, 14 Feb 2002, Andrea Arcangeli wrote:
> >
> > I've recently changed the code to make it *not* call unmap_kiobuf/vfree
> > from irq context.  Instead bttv 0.8.x doesn't allow you to close the
> > device with DMA xfers in flight.  If you try this the release() fops
> > handler will block until the transfer is done, then unmap_kiobuf from
> > process context, then return.
>
> perfect, that's the right fix for 2.4 (waiting DMA to complete at
> ->release looks also much saner).

I think it's the right fix regardless. We should do as little as possible
from irq contexts, and that holds _doubly_ true if it mucks with things
like the page cache.

> With aio in 2.5 we may want to change this property for the unpinning
> stage that would be better run asynchronously from irq handlers, but I
> wouldn't change that for 2.4 (at least until we're forced to ship aio in
> production on top 2.4, that cannot happen until a final user<->kernel API is
> registered somewhere).

I'd really really hate to have the IO make pages go away from irq context
in 2.5.x too. I think async IO should always be started and cleaned up in
a user context - there simply isn't any reason not to (the notion of doing
an exit() or execve() with IO still pending to now-dead-memory is rather
horrible in itself).

> I think the foundamental design mistake that leads to __free_pages to
> fail from irq, is that we allow an anonymous page to reach count 0 and to be
> still in the LRU (the count == 0 check in shrink_cache is the other side
> of the hack too). That's the real BUG, that breaks subtly the freelist
> semantics

Agreed. We should NEVER free the pages from the irq.

		Linus