On Tue, May 09, 2023 at 02:43:19PM -0700, Darrick J. Wong wrote: > On Tue, May 09, 2023 at 02:12:41PM -0700, Lorenzo Stoakes wrote: > > On Tue, May 09, 2023 at 01:46:09PM -0700, Christoph Hellwig wrote: > > > On Tue, May 09, 2023 at 12:56:32PM -0400, Kent Overstreet wrote: > > > > From: Kent Overstreet <kent.overstreet@xxxxxxxxx> > > > > > > > > This is needed for bcachefs, which dynamically generates per-btree node > > > > unpack functions. > > > > > > No, we will never add back a way for random code allocating executable > > > memory in kernel space. > > > > Yeah I think I glossed over this aspect a bit as it looks ostensibly like simply > > reinstating a helper function because the code is now used in more than one > > place (at lsf/mm so a little distracted :) > > > > But it being exported is a problem. Perhaps there's another way of acheving the > > same aim without having to do so? > > I already trolled Kent with this on IRC, but for the parts of bcachefs > that want better assembly code than whatever gcc generates from the C > source, could you compile code to BPF and then let the BPF JIT engines > turn that into machine code for you? It's an intriguing idea, but it'd be a _lot_ of work and this is old code that's never had a single bug - I'm not in a hurry to rewrite it. And there would still be the issue that we've still got lots of little unpack functions that go with other tables; we can't just burn a full page per unpack function, that would waste way too much memory, and if we put them together then we're stuck writing a whole nother allocator - nope, and then we're also mucking with the memory layout of the data structures used in the very hottest paths in the filesystem - I'm very wary of introducing performance regressions there. I think it'd be much more practical to find some way of making vmalloc_exec() more palatable. What are the exact concerns?
