Messages in this thread | | | Date | Fri, 10 May 2024 11:15:59 +0200 | Subject | Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions | From | David Hildenbrand <> |
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On 09.05.24 13:05, Vincent Donnefort wrote: > On Tue, May 07, 2024 at 10:34:02PM -0400, Steven Rostedt wrote: >> On Tue, 30 Apr 2024 12:13:51 +0100 >> Vincent Donnefort <vdonnefort@google.com> wrote: >> >>> +#ifdef CONFIG_MMU >>> +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer, >>> + struct vm_area_struct *vma) >>> +{ >>> + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff; >>> + unsigned int subbuf_pages, subbuf_order; >>> + struct page **pages; >>> + int p = 0, s = 0; >>> + int err; >>> + >>> + /* Refuse MP_PRIVATE or writable mappings */ >>> + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC || >>> + !(vma->vm_flags & VM_MAYSHARE)) >>> + return -EPERM; >>> + >>> + /* >>> + * Make sure the mapping cannot become writable later. Also tell the VM >>> + * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND). Finally, >>> + * prevent migration, GUP and dump (VM_IO). >>> + */ >>> + vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE); >> >> Do we really need the VM_IO? >> >> When testing this in gdb, I would get: >> >> (gdb) p tmap->map->subbuf_size >> Cannot access memory at address 0x7ffff7fc2008 >> >> It appears that you can't ptrace IO memory. When I removed that flag, >> gdb has no problem reading that memory. > > Yeah, VM_IO indeed implies DONTDUMP. VM_IO was part of Linus recommendations.
Yes, the VM should recognize that memory to some degree as being special already due to VM_MIXEDMAP and VM_DONTEXPAND.
#define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)
So any of these flag achieve that (e.g., mlock_fixup() checks VM_SPECIAL). KSM similarly skips VM_DONTEXPAND and VM_MIXEDMAP (likely we should be using VM_SPECIAL in vma_ksm_compatible()). Not sure about page migration, likely its fine.
Thinking about MADV_DONTNEED, I can spot in madvise_dontneed_free_valid_vma() only that we disallow primarily VM_PFNMAP.
.. I assume if user space MADV_DONTNEED's some pages we'll simply get a page fault later on access that will SIGBUS, handling that gracefully (we should double-check!).
> But perhaps, VM_DONTEXPAND and MIXEDMAP (implicitely set by vm_insert_pages) are > enough protection?
Do we want to dump these pages? VM_DONTDUMP might be reasonabe then.
> > I don't see how anything could use GUP there and as David pointed-out on the > previous version, it doesn't event prevent the GUP-fast path.
Yes, GUP-fast would still have worked under some conditions.
-- Cheers,
David / dhildenb
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