Re: [PATCH bpf-next 08/13] uprobes/x86: Add support to optimize uprobes

Oleg Nesterov
2024-12-16T10:13:35Z

David,

let me say first that my understanding of this magic is very limited,
please correct me.

On 12/16, David Laight wrote:
>
> It all depends on how hard __replace_page() tries to be atomic.
> The page has to change from one backed by the executable to a private
> one backed by swap - otherwise you can't write to it.

This is what uprobe_write_opcode() does,

> But the problems arise when the instruction prefetch unit has read
> part of the 5-byte instruction (it might even only read half a cache
> line at a time).
> I'm not sure how long the pipeline can sit in that state - but I
> can do a memory read of a PCIe address that takes ~3000 clocks.
> (And a misaligned AVX-512 read is probably eight 8-byte transfers.)
>
> So I think you need to force an interrupt while the PTE is invalid.
> And that need to be simultaneous on all cpu running that process.

__replace_page() does ptep_get_and_clear(old_pte) + flush_tlb_page().

That's not enough?

> Stopping the process using ptrace would do it.

Not an option :/

Oleg.