[PATCH 2/2] MIPS: mm: do not write a huge TLB entry when the probe misses

[Orgad Shaneh]

__update_tlb() probes for the 8K pair at the address it is given and,
for a huge pmd, writes the huge entry with tlbwi at the probed index or
with tlbwr when the probe misses. That works only if the caller passes
the faulting address, so that the probe finds the 4K entry the refill
handler loaded for the faulting page and the huge entry replaces it.

update_mmu_cache_pmd() is not called that way. do_set_pmd() has always
passed the huge-aligned address, and since commit ebcfc63d6bca ("mm:
abstract THP allocation") the anonymous THP fault path does too
(map_anon_folio_pmd()). The probe then misses the stale 4K entry, which
sits at the faulting page somewhere else in the 2 MB range, tlbwr adds
a huge entry next to it, and the TLB holds two entries matching the
faulting address. Octeon raises "Machine Check exception - caused by
multiple matching entries in the TLB" on the next refill of that page;
a process on a CN63XX board died this way within a second of start, on
the first anonymous THP of its bss.

Skip the write when the probe misses. The refill and TLBL/TLBS handlers
probe the faulting address themselves and rewrite the stale entry with
the huge one (they also set the software young bit), so the entry is
installed on the next access at the cost of one exception.

Fixes: fd062c847a8c ("MIPS: TLB support for hugetlbfs.")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-5
Signed-off-by: Orgad Shaneh <orgads@xxxxxxxxx>
---
diff --git a/arch/mips/mm/tlb-r4k.c b/arch/mips/mm/tlb-r4k.c
--- a/arch/mips/mm/tlb-r4k.c
+++ b/arch/mips/mm/tlb-r4k.c
@@ -332,6 +332,19 @@ void __update_tlb(struct vm_area_struct * vma, unsigned long address, pte_t pte)
 	/* this could be a huge page  */
 	if (pmd_leaf(*pmdp)) {
 		unsigned long lo;
+
+		/*
+		 * The probe above only covers the 8K pair at @address, and
+		 * a huge mapping is installed with the huge-aligned address
+		 * while the refill that started the fault left a 4K entry
+		 * for the faulting page elsewhere in the range. Writing the
+		 * huge entry to a random index would leave two entries
+		 * matching the faulting address; leave it to the refill and
+		 * TLBL/TLBS handlers, which probe the faulting address.
+		 */
+		if (idx < 0)
+			goto out;
+
 		write_c0_pagemask(PM_HUGE_MASK);
 		ptep = (pte_t *)pmdp;
 		lo = pte_to_entrylo(pte_val(*ptep));
@@ -339,10 +352,7 @@ void __update_tlb(struct vm_area_struct * vma, unsigned long address, pte_t pte)
 		write_c0_entrylo1(lo + (HPAGE_SIZE >> 7));
 
 		mtc0_tlbw_hazard();
-		if (idx < 0)
-			tlb_write_random();
-		else
-			tlb_write_indexed();
+		tlb_write_indexed();
 		tlbw_use_hazard();
 		write_c0_pagemask(PM_DEFAULT_MASK);
 	} else
@@ -380,6 +390,7 @@ void __update_tlb(struct vm_area_struct * vma, unsigned long address, pte_t pte)
 			tlb_write_indexed();
 	}
 	tlbw_use_hazard();
+out:
 	htw_start();
 	flush_micro_tlb_vm(vma);
 
-- 
2.47.0