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

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On Wed, Jul 06, 2022 at 07:40:44PM +0100, Matthew Wilcox wrote:
> On Wed, Jul 06, 2022 at 11:36:19AM -0700, Alexei Starovoitov wrote:
> > 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.
> 
> I did read it.  It says
> 
> : The only limit known to the user space is BPF_MAXINSNS (4096). It’s the
> : maximum number of instructions that the unprivileged bpf program can have.
> 
> It really seems pretty clear to me.  You're saying my understanding
> is wrong.  So it must be badly written.  Even now, I don't understand
> how I've misunderstood it.

huh? Still missing 'unprivileged' in the above ?
and completely ignoring the rest of the paragraph in the link above?

> > > > 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.
> 
> Really, this is not a productive way to respond.  I want to help
> and you're just snarling at me.

To help you need to understand what bpf is while you're failing
to read the doc.




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