@@ -632,7 +632,7 @@ struct mm_struct {
atomic_t tlb_flush_pending;
#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
/* See flush_tlb_batched_pending() */
- bool tlb_flush_batched;
+ atomic_t tlb_flush_batched;
#endif
struct uprobes_state uprobes_state;
#ifdef CONFIG_PREEMPT_RT
@@ -621,9 +621,20 @@ void try_to_unmap_flush_dirty(void)
try_to_unmap_flush();
}
+/*
+ * The 0-14 bit of mm->tlb_flush_batched records pending generations,
+ * and the 16-30 bit records flushed generations.
+ */
+#define TLB_FLUSH_BATCH_FLUSHED_SHIFT 16
+#define TLB_FLUSH_BATCH_PENDING_MASK \
+ ((1 << (TLB_FLUSH_BATCH_FLUSHED_SHIFT - 1)) - 1)
+#define TLB_FLUSH_BATCH_PENDING_LARGE \
+ (TLB_FLUSH_BATCH_PENDING_MASK / 2)
+
static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
{
struct tlbflush_unmap_batch *tlb_ubc = ¤t->tlb_ubc;
+ int batch, nbatch;
arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
tlb_ubc->flush_required = true;
@@ -633,7 +644,22 @@ static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
* before the PTE is cleared.
*/
barrier();
- mm->tlb_flush_batched = true;
+ batch = atomic_read(&mm->tlb_flush_batched);
+retry:
+ if ((batch & TLB_FLUSH_BATCH_PENDING_MASK) > TLB_FLUSH_BATCH_PENDING_LARGE) {
+ /*
+ * To avoid ""pending"" to catch up with ""flushed""
+ * because of overflow. Reset "pending" and "flushed"
+ * to be "1" and "0" if "pending" becomes large.
+ */
+ nbatch = atomic_cmpxchg(&mm->tlb_flush_batched, batch, 1);
+ if (nbatch != batch) {
+ batch = nbatch;
+ goto retry;
+ }
+ } else {
+ atomic_inc(&mm->tlb_flush_batched);
+ }
/*
* If the PTE was dirty then it's best to assume it's writable. The
@@ -680,15 +706,19 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
*/
void flush_tlb_batched_pending(struct mm_struct *mm)
{
- if (data_race(mm->tlb_flush_batched)) {
- flush_tlb_mm(mm);
+ int batch = atomic_read(&mm->tlb_flush_batched);
+ int pending = batch & TLB_FLUSH_BATCH_PENDING_MASK;
+ int flushed = batch >> TLB_FLUSH_BATCH_FLUSHED_SHIFT;
+ if (pending != flushed) {
+ flush_tlb_mm(mm);
/*
- * Do not allow the compiler to re-order the clearing of
- * tlb_flush_batched before the tlb is flushed.
+ * If the new TLB flushing is pended during flushing,
+ * leave mm->tlb_flush_batched as is, to avoid to lose
+ * flushing.
*/
- barrier();
- mm->tlb_flush_batched = false;
+ atomic_cmpxchg(&mm->tlb_flush_batched, batch,
+ pending | (pending << TLB_FLUSH_BATCH_FLUSHED_SHIFT));
}
}
#else
In theory, the following race is possible for batched TLB flushing. CPU0 CPU1 ---- ---- shrink_page_list() unmap zap_pte_range() flush_tlb_batched_pending() flush_tlb_mm() try_to_unmap() set_tlb_ubc_flush_pending() mm->tlb_flush_batched = true mm->tlb_flush_batched = false After the TLB is flushed on CPU1 via flush_tlb_mm() and before mm->tlb_flush_batched is set to false, some PTE is unmapped on CPU0 and the TLB flushing is pended. Then the pended TLB flushing will be lost. Although both set_tlb_ubc_flush_pending() and flush_tlb_batched_pending() are called with PTL locked, different PTL instances may be used. Because the race window is really small, and the lost TLB flushing will cause problem only if a TLB entry is inserted before the unmapping in the race window, the race is only theoretical. But the fix is simple and cheap too. Syzbot has reported this too as follows, ================================================================== BUG: KCSAN: data-race in flush_tlb_batched_pending / try_to_unmap_one write to 0xffff8881072cfbbc of 1 bytes by task 17406 on cpu 1: flush_tlb_batched_pending+0x5f/0x80 mm/rmap.c:691 madvise_free_pte_range+0xee/0x7d0 mm/madvise.c:594 walk_pmd_range mm/pagewalk.c:128 [inline] walk_pud_range mm/pagewalk.c:205 [inline] walk_p4d_range mm/pagewalk.c:240 [inline] walk_pgd_range mm/pagewalk.c:277 [inline] __walk_page_range+0x981/0x1160 mm/pagewalk.c:379 walk_page_range+0x131/0x300 mm/pagewalk.c:475 madvise_free_single_vma mm/madvise.c:734 [inline] madvise_dontneed_free mm/madvise.c:822 [inline] madvise_vma mm/madvise.c:996 [inline] do_madvise+0xe4a/0x1140 mm/madvise.c:1202 __do_sys_madvise mm/madvise.c:1228 [inline] __se_sys_madvise mm/madvise.c:1226 [inline] __x64_sys_madvise+0x5d/0x70 mm/madvise.c:1226 do_syscall_x64 arch/x86/entry/common.c:50 [inline] do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x44/0xae write to 0xffff8881072cfbbc of 1 bytes by task 71 on cpu 0: set_tlb_ubc_flush_pending mm/rmap.c:636 [inline] try_to_unmap_one+0x60e/0x1220 mm/rmap.c:1515 rmap_walk_anon+0x2fb/0x470 mm/rmap.c:2301 try_to_unmap+0xec/0x110 shrink_page_list+0xe91/0x2620 mm/vmscan.c:1719 shrink_inactive_list+0x3fb/0x730 mm/vmscan.c:2394 shrink_list mm/vmscan.c:2621 [inline] shrink_lruvec+0x3c9/0x710 mm/vmscan.c:2940 shrink_node_memcgs+0x23e/0x410 mm/vmscan.c:3129 shrink_node+0x8f6/0x1190 mm/vmscan.c:3252 kswapd_shrink_node mm/vmscan.c:4022 [inline] balance_pgdat+0x702/0xd30 mm/vmscan.c:4213 kswapd+0x200/0x340 mm/vmscan.c:4473 kthread+0x2c7/0x2e0 kernel/kthread.c:327 ret_from_fork+0x1f/0x30 value changed: 0x01 -> 0x00 Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 71 Comm: kswapd0 Not tainted 5.16.0-rc1-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 ================================================================== Signed-off-by: "Huang, Ying" <ying.huang@intel.com> Reported-by: syzbot+aa5bebed695edaccf0df@syzkaller.appspotmail.com Cc: Nadav Amit <namit@vmware.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Will Deacon <will@kernel.org> Cc: Yu Zhao <yuzhao@google.com> Cc: Marco Elver <elver@google.com> --- include/linux/mm_types.h | 2 +- mm/rmap.c | 44 +++++++++++++++++++++++++++++++++------- 2 files changed, 38 insertions(+), 8 deletions(-)