[PATCH] arm64: hugetlb: Prevent write fault livelocks on stale TLB entries

[Chris Down <chris@chrisdown.name>]

In Meta production, we have had tasks get stuck in a fault loop while
using hugetlb on arm64. During my investigations I found that affected
tasks share a private hugetlb page.

Looking at the stuck task states, I found the following ordering:

  CPU0                                 CPU1
  ----                                 ----
  read page
  [read only entry cached in TLB]
                                       write page
                                       hugetlb_fault()
                                         hugetlb_wp()
                                         [reuse folio]
                                         set_huge_ptep_maybe_writable()
                                           set_huge_ptep_writable()
                                         [flush local TLB only]

  write page using stale entry
  hugetlb_fault()
    huge_ptep_set_access_flags()
    [entry already writable, no flush]
  retry write using stale entry
  [fault indefinitely. well, that's not ideal!]

The reason is that when a write fault makes a non-contiguous hugetlb
entry writable, it only invalidates the local TLB. However, another CPU
can keep the old read-only entry, so its write will fault even though
the page table already permits it. hugetlb_fault() then finds nothing to
change and leaves the stale entry in place, so the same write keeps
going round in circles.

This is particularly bad because, if the kernel is copying to user
memory, the retry stays inside the kernel and the task can't act on the
pending signal.

It can even remain unkillable while holding locks across the copy,
resulting in further failures. For example, a read into a hugetlb
buffer can hold the pipe mutex and block other users of the pipe until
the loop ends, causing cascading lockups across the machine.

To fix this, when a write fault finds nothing to change in a
non-contiguous hugetlb entry, let's invalidate the local entry.

Fixes: cb1fa2e99955 ("arm64, tlbflush: don't TLBI broadcast if page reused in write fault")
Cc: <stable@vger.kernel.org>
Signed-off-by: Chris Down <chris@chrisdown.name>
---
 arch/arm64/mm/hugetlbpage.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index 8e799c1fe0aa..ff1f53e435e1 100644
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -429,9 +429,24 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
 	VM_WARN_ON(!pte_present(pte));
 	ncontig = num_contig_ptes(huge_page_size(hstate_vma(vma)), &pgsize);
 
-	if (!pte_cont(pte))
-		return __ptep_set_access_flags_anysz(vma, addr, ptep, pte,
-						     dirty, pgsize);
+	if (!pte_cont(pte)) {
+		if (__ptep_set_access_flags_anysz(vma, addr, ptep, pte,
+						  dirty, pgsize))
+			return 1;
+
+		/*
+		 * Another CPU may have made the entry writable without
+		 * invalidating our stale read-only TLB entry. To avoid
+		 * indefinitely faulting on this entry, flush it.
+		 */
+		if (dirty)
+			__flush_hugetlb_tlb_range(vma, addr, addr + pgsize,
+						  pgsize,
+						  TLBF_NOWALKCACHE |
+						  TLBF_NOBROADCAST |
+						  TLBF_NONOTIFY);
+		return 0;
+	}
 
 	if (!__cont_access_flags_changed(ptep, pte, ncontig))
 		return 0;

base-commit: 067c5c24cef23d913b67eaaf71ae67e7169a6620
-- 
2.54.0