On Wed, Mar 1, 2023 at 4:54 PM Hyeonggon Yoo [off-list ref] wrote:

On Wed, Mar 01, 2023 at 10:42:48AM -0800, Suren Baghdasaryan wrote:

quoted

On Wed, Mar 1, 2023 at 10:34 AM Suren Baghdasaryan [off-list ref] wrote:

quoted

On Tue, Feb 28, 2023 at 11:57 PM Hyeonggon Yoo [off-list ref] wrote:

quoted

On Wed, Mar 01, 2023 at 07:43:33AM +0000, Hyeonggon Yoo wrote:

quoted

On Mon, Feb 27, 2023 at 09:36:17AM -0800, Suren Baghdasaryan wrote:

quoted

Write-locking VMAs before isolating them ensures that page fault
handlers don't operate on isolated VMAs.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/mmap.c  | 1 +
 mm/nommu.c | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 1f42b9a52b9b..f7ed357056c4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c

@@ -2255,6 +2255,7 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 static inline int munmap_sidetree(struct vm_area_struct *vma,
                               struct ma_state *mas_detach)
 {
+   vma_start_write(vma);
    mas_set_range(mas_detach, vma->vm_start, vma->vm_end - 1);

I may be missing something, but have few questions:

      1) Why does a writer need to both write-lock a VMA and mark the VMA detached
         when unmapping it, isn't it enough to just only write-lock a VMA?

We need to mark the VMA detached to avoid handling page fault in a
detached VMA. The possible scenario is:

lock_vma_under_rcu
  vma = mas_walk(&mas)
                                                        munmap_sidetree
                                                          vma_start_write(vma)

mas_store_gfp() // remove VMA from the tree
                                                          vma_end_write_all()
  vma_start_read(vma)
  // we locked the VMA but it is not part of the tree anymore.

So, marking the VMA locked before vma_end_write_all() and checking

Sorry, I should have said "marking the VMA *detached* before
vma_end_write_all() and checking vma->detached after vma_start_read()
helps us avoid handling faults in the detached VMA."

quoted

vma->detached after vma_start_read() helps us avoid handling faults in
the detached VMA.

Thank you for explanation, that makes sense!

By the way, if there are no 32bit users of Per-VMA lock (are there?),
"detached" bool could be a VMA flag (i.e. making it depend on 64BIT
and selecting ARCH_USES_HIGH_VMA_FLAGS)

Yeah, I thought about it but didn't want to make assumptions about
potential users just yet. Besides, I heard there are attempts to make
vm_flags to be always 64-bit (I think Matthew mentioned that to me
once). If that happens, we won't need any dependencies here. Either
way, this conversion into a flag can be done as an additional
optimization later on. I prefer to keep the main patchset as simple as
possible for now.
Thanks,
Suren.

Thanks,
Hyeonggon

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