On Tue, May 16, 2023 at 5:35 AM David Hildenbrand david@redhat.com wrote:
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@m.... 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?
The idea would be that T2's arch_swap_readpage() could potentially not find tags if it ran after swap_free(), so T2 would produce a page without restored tags. But that wouldn't matter, because T1 reaching swap_free() means that T2 will follow the goto at [1] after waiting for T1 to unlock at [2], and T2's page will be discarded.
This might be a naive question, but how does MTE play along with shared anonymous pages?
It should work fine. shmem_writepage() calls swap_writepage() which calls arch_prepare_to_swap() to write the tags. And shmem_swapin_folio() has a call to arch_swap_restore() to restore them.
Peter
[1] https://github.com/torvalds/linux/blob/f1fcbaa18b28dec10281551dfe6ed3a3ed80e... [2] https://github.com/torvalds/linux/blob/f1fcbaa18b28dec10281551dfe6ed3a3ed80e...