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

[Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>]

On Wed, Mar 6, 2024 at 5:13 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Mar 6, 2024 at 1:46 PM Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> 
> 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.

We still need to get to PTE level to use set_pte_at(). So, either
store it in task_struct for faster PF handling, or add another
non-sleeping vmap function that will do something like this:

vm_area_set_page_at(addr, page)
{
   pgd = pgd_offset_k(addr)
   p4d = vunmap_p4d_range(pgd, addr)
   pud = pud_offset(p4d, addr)
   pmd = pmd_offset(pud, addr)
   pte = pte_offset_kernel(pmd, addr)

  set_pte_at(init_mm, addr, pte, mk_pte(page...));
}

Pasha