Re: [PATCH v4 bpf-next 2/2] mm: Introduce VM_SPARSE kind and vm_area_[un]map_pages().

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

On Wed, Mar 6, 2024 at 1:46 PM Pasha Tatashin <pasha.tatashin@soleen.com> wrote:
>
> > > This interface and in general VM_SPARSE would be useful for
> > > dynamically grown kernel stacks [1]. However, the might_sleep() here
> > > would be a problem. We would need to be able to handle
> > > vm_area_map_pages() from interrupt disabled context therefore no
> > > sleeping. The caller would need to guarantee that the page tables are
> > > pre-allocated before the mapping.
> >
> > Sounds like we'd need to differentiate two kinds of sparse regions.
> > One that is really sparse where page tables are not populated (bpf use case)
> > and another where only the pte level might be empty.
> > Only the latter one will be usable for such auto-grow stacks.
> >
> > Months back I played with this idea:
> > https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?&id=ce63949a879f2f26c1c1834303e6dfbfb79d1fbd
> > that
> > "Make vmap_pages_range() allocate page tables down to the last (PTE) level."
> > Essentially pass NULL instead of 'pages' into vmap_pages_range()
> > and it will populate all levels except the last.
>
> Yes, this is what is needed, however, it can be a little simpler with
> kernel stacks:
> given that the first page in the vm_area is mapped when stack is first
> allocated, and that the VA range is aligned to 16K, we actually are
> guaranteed to have all page table levels down to pte pre-allocated
> during that initial mapping. Therefore, we do not need to worry about
> allocating them later during PFs.

Ahh. Found:
stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN, ...

> > Then the page fault handler can service a fault in auto-growing stack
> > area if it has a page stashed in some per-cpu free list.
> > I suspect this is something you might need for
> > "16k stack that is populated on fault",
> > plus a free list of 3 pages per-cpu,
> > and set_pte_at() in pf handler.
>
> Yes, what you described is exactly what I am working on: using 3-pages
> per-cpu to handle kstack page faults. The only thing that is missing
> is that I would like to have the ability to call a non-sleeping
> version of vm_area_map_pages().

vm_area_map_pages() cannot be non-sleepable, since the [start, end)
range will dictate whether mid level allocs and locks are needed.

Instead in alloc_thread_stack_node() you'd need a flavor
of get_vm_area() that can align the range to THREAD_ALIGN.
Then immediately call _sleepable_ vm_area_map_pages() to populate
the first page and later set_pte_at() the other pages on demand
from the fault handler.