On Thu, Mar 14, 2024 at 07:43:06PM +0000, Matthew Wilcox wrote: > On Tue, Mar 12, 2024 at 10:18:10AM -0700, H. Peter Anvin wrote: > > Second, non-dynamic kernel memory is one of the core design decisions in > > Linux from early on. This means there are lot of deeply embedded assumptions > > which would have to be untangled. > > I think there are other ways of getting the benefit that Pasha is seeking > without moving to dynamically allocated kernel memory. One icky thing > that XFS does is punt work over to a kernel thread in order to use more > stack! That breaks a number of things including lockdep (because the > kernel thread doesn't own the lock, the thread waiting for the kernel > thread owns the lock). > > If we had segmented stacks, XFS could say "I need at least 6kB of stack", > and if less than that was available, we could allocate a temporary > stack and switch to it. I suspect Google would also be able to use this > API for their rare cases when they need more than 8kB of kernel stack. > Who knows, we might all be able to use such a thing. > > I'd been thinking about this from the point of view of allocating more > stack elsewhere in kernel space, but combining what Pasha has done here > with this idea might lead to a hybrid approach that works better; allocate > 32kB of vmap space per kernel thread, put 12kB of memory at the top of it, > rely on people using this "I need more stack" API correctly, and free the > excess pages on return to userspace. No complicated "switch stacks" API > needed, just an "ensure we have at least N bytes of stack remaining" API. Why would we need an "I need more stack" API? Pasha's approach seems like everything we need for what you're talking about.
