Re: [PATCH bpf-next 0/5] bpf: BPF specific memory allocator.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Jul 06, 2022 at 07:31:34PM +0100, Matthew Wilcox wrote:
> On Wed, Jul 06, 2022 at 11:26:35AM -0700, Alexei Starovoitov wrote:
> > On Wed, Jul 06, 2022 at 07:21:29PM +0100, Matthew Wilcox wrote:
> > > On Wed, Jul 06, 2022 at 11:05:25AM -0700, Alexei Starovoitov wrote:
> > > > On Wed, Jul 06, 2022 at 06:55:36PM +0100, Matthew Wilcox wrote:
> > > > > For example, I assume that a BPF program
> > > > > has a fairly tight limit on how much memory it can cause to be allocated.
> > > > > Right?
> > > > 
> > > > No. It's constrained by memcg limits only. It can allocate gigabytes.
> > > 
> > > I'm confused.  A BPF program is limited to executing 4096 insns and
> > > using a maximum of 512 bytes of stack space, but it can allocate an
> > > unlimited amount of heap?  That seems wrong.
> > 
> > 4k insn limit was lifted years ago.
> 
> You might want to update the documentation.
> https://www.kernel.org/doc/html/latest/bpf/bpf_design_QA.html
> still says 4096.

No. Please read what you're quoting first.

> > bpf progs are pretty close to be at parity with kernel modules.
> > Except that they are safe, portable, and users have full visibility into them.
> > It's not a blob of bytes unlike .ko.
> 
> It doesn't seem unreasonable to expect them to behave like kernel
> modules, then.  If they want to allocate memory in NMI context, then
> they should get to preallocate it before they go into NMI context.

You're still missing 'any context' part from the previous email.