Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before swap_free()
From: David Hildenbrand
Date: Tue May 16 2023 - 08:36:22 EST
On 16.05.23 01:40, Peter Collingbourne wrote:
On Mon, May 15, 2023 at 06:34:30PM +0100, Catalin Marinas wrote:
On Sat, May 13, 2023 at 05:29:53AM +0200, David Hildenbrand wrote:
On 13.05.23 01:57, Peter Collingbourne wrote:
diff --git a/mm/memory.c b/mm/memory.c
index 01a23ad48a04..83268d287ff1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3914,19 +3914,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
}
}
- /*
- * Remove the swap entry and conditionally try to free up the swapcache.
- * We're already holding a reference on the page but haven't mapped it
- * yet.
- */
- swap_free(entry);
- if (should_try_to_free_swap(folio, vma, vmf->flags))
- folio_free_swap(folio);
-
- inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
- dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
pte = mk_pte(page, vma->vm_page_prot);
-
/*
* Same logic as in do_wp_page(); however, optimize for pages that are
* certainly not shared either because we just allocated them without
@@ -3946,8 +3934,21 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
pte = pte_mksoft_dirty(pte);
if (pte_swp_uffd_wp(vmf->orig_pte))
pte = pte_mkuffd_wp(pte);
+ arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
vmf->orig_pte = pte;
+ /*
+ * Remove the swap entry and conditionally try to free up the swapcache.
+ * We're already holding a reference on the page but haven't mapped it
+ * yet.
+ */
+ swap_free(entry);
+ if (should_try_to_free_swap(folio, vma, vmf->flags))
+ folio_free_swap(folio);
+
+ inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
+ dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
+
/* ksm created a completely new copy */
if (unlikely(folio != swapcache && swapcache)) {
page_add_new_anon_rmap(page, vma, vmf->address);
@@ -3959,7 +3960,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
VM_BUG_ON(!folio_test_anon(folio) ||
(pte_write(pte) && !PageAnonExclusive(page)));
set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
- arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
folio_unlock(folio);
if (folio != swapcache && swapcache) {
You are moving the folio_free_swap() call after the folio_ref_count(folio)
== 1 check, which means that such (previously) swapped pages that are
exclusive cannot be detected as exclusive.
There must be a better way to handle MTE here.
Where are the tags stored, how is the location identified, and when are they
effectively restored right now?
I haven't gone through Peter's patches yet but a pretty good description
of the problem is here:
https://lore.kernel.org/all/5050805753ac469e8d727c797c2218a9d780d434.camel@xxxxxxxxxxxx/.
I couldn't reproduce it with my swap setup but both Qun-wei and Peter
triggered it.
In order to reproduce this bug it is necessary for the swap slot cache
to be disabled, which is unlikely to occur during normal operation. I
was only able to reproduce the bug by disabling it forcefully with the
following patch:
diff --git a/mm/swap_slots.c b/mm/swap_slots.c
index 0bec1f705f8e0..25afba16980c7 100644
--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -79,7 +79,7 @@ void disable_swap_slots_cache_lock(void)
static void __reenable_swap_slots_cache(void)
{
- swap_slot_cache_enabled = has_usable_swap();
+ swap_slot_cache_enabled = false;
}
void reenable_swap_slots_cache_unlock(void)
With that I can trigger the bug on an MTE-utilizing process by running
a program that enumerates the process's private anonymous mappings and
calls process_madvise(MADV_PAGEOUT) on all of them.
When a tagged page is swapped out, the arm64 code stores the metadata
(tags) in a local xarray indexed by the swap pte. When restoring from
swap, the arm64 set_pte_at() checks this xarray using the old swap pte
and spills the tags onto the new page. Apparently something changed in
the kernel recently that causes swap_range_free() to be called before
set_pte_at(). The arm64 arch_swap_invalidate_page() frees the metadata
from the xarray and the subsequent set_pte_at() won't find it.
If we have the page, the metadata can be restored before set_pte_at()
and I guess that's what Peter is trying to do (again, I haven't looked
at the details yet; leaving it for tomorrow).
Is there any other way of handling this? E.g. not release the metadata
in arch_swap_invalidate_page() but later in set_pte_at() once it was
restored. But then we may leak this metadata if there's no set_pte_at()
(the process mapping the swap entry died).
Another problem that I can see with this approach is that it does not
respect reference counts for swap entries, and it's unclear whether that
can be done in a non-racy fashion.
Another approach that I considered was to move the hook to swap_readpage()
as in the patch below (sorry, it only applies to an older version
of Android's android14-6.1 branch and not mainline, but you get the
idea). But during a stress test (running the aforementioned program that
calls process_madvise(MADV_PAGEOUT) in a loop during an Android "monkey"
test) I discovered the following racy use-after-free that can occur when
two tasks T1 and T2 concurrently restore the same page:
T1: | T2:
arch_swap_readpage() |
| arch_swap_readpage() -> mte_restore_tags() -> xe_load()
swap_free() |
| arch_swap_readpage() -> mte_restore_tags() -> mte_restore_page_tags()
We can avoid it by taking the swap_info_struct::lock spinlock in
mte_restore_tags(), but it seems like it would lead to lock contention.
Would the idea be to fail swap_readpage() on the one that comes last,
simply retrying to lookup the page?
This might be a naive question, but how does MTE play along with shared
anonymous pages?
--
Thanks,
David / dhildenb