Date:	Fri, 17 Jun 2016 11:30:03 +0900
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	linux-mm@...ck.org
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Michal Hocko <mhocko@...e.cz>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: [PATCH v1 1/2] mm: thp: move pmd check inside ptl for freeze_page()

I found a race condition triggering VM_BUG_ON() in freeze_page(), when running
a testcase with 3 processes:
  - process 1: keep writing thp,
  - process 2: keep clearing soft-dirty bits from virtual address of process 1
  - process 3: call migratepages for process 1,

The kernel message is like this:

  kernel BUG at /src/linux-dev/mm/huge_memory.c:3096!
  invalid opcode: 0000 [#1] SMP
  Modules linked in: cfg80211 rfkill crc32c_intel ppdev serio_raw pcspkr virtio_balloon virtio_console parport_pc parport pvpanic acpi_cpufreq tpm_tis tpm i2c_piix4 virtio_blk virtio_net ata_generic pata_acpi floppy virtio_pci virtio_ring virtio
  CPU: 0 PID: 28863 Comm: migratepages Not tainted 4.6.0-v4.6-160602-0827-+ #2
  Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
  task: ffff880037320000 ti: ffff88007cdd0000 task.ti: ffff88007cdd0000
  RIP: 0010:[<ffffffff811f8e06>]  [<ffffffff811f8e06>] split_huge_page_to_list+0x496/0x590
  RSP: 0018:ffff88007cdd3b70  EFLAGS: 00010202
  RAX: 0000000000000001 RBX: ffff88007c7b88c0 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: 0000000700000200 RDI: ffffea0003188000
  RBP: ffff88007cdd3bb8 R08: 0000000000000001 R09: 00003ffffffff000
  R10: ffff880000000000 R11: ffffc000001fffff R12: ffffea0003188000
  R13: ffffea0003188000 R14: 0000000000000000 R15: 0400000000000080
  FS:  00007f8ec241d740(0000) GS:ffff88007dc00000(0000) knlGS:0000000000000000             CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007f8ec1f3ed20 CR3: 000000003707b000 CR4: 00000000000006f0
  Stack:
   ffffffff8139ef6d ffffea00031c6280 ffff88011ffec000 0000000000000000
   0000700000400000 0000700000200000 ffff88007cdd3d08 ffff8800dbbe3008
   0400000000000080 ffff88007cdd3c20 ffffffff811dd0b1 ffff88007cdd3d68
  Call Trace:
   [<ffffffff8139ef6d>] ? list_del+0xd/0x30
   [<ffffffff811dd0b1>] queue_pages_pte_range+0x4d1/0x590
   [<ffffffff811ca1a4>] __walk_page_range+0x204/0x4e0
   [<ffffffff811ca4f1>] walk_page_range+0x71/0xf0
   [<ffffffff811db935>] queue_pages_range+0x75/0x90
   [<ffffffff811dcbe0>] ? queue_pages_hugetlb+0x190/0x190
   [<ffffffff811dca50>] ? new_node_page+0xc0/0xc0
   [<ffffffff811ddac0>] ? change_prot_numa+0x40/0x40
   [<ffffffff811dc001>] migrate_to_node+0x71/0xd0
   [<ffffffff811ddd73>] do_migrate_pages+0x1c3/0x210
   [<ffffffff811de0b1>] SyS_migrate_pages+0x261/0x290
   [<ffffffff816f53f2>] entry_SYSCALL_64_fastpath+0x1a/0xa4
  Code: e8 b0 87 fb ff 0f 0b 48 c7 c6 30 32 9f 81 e8 a2 87 fb ff 0f 0b 48 c7 c6 b8 46 9f 81 e8 94 87 fb ff 0f 0b 85 c0 0f 84 3e fd ff ff <0f> 0b 85 c0 0f 85 a6 00 00 00 48 8b 75 c0 4c 89 f7 41 be f0 ff
  RIP  [<ffffffff811f8e06>] split_huge_page_to_list+0x496/0x590
   RSP <ffff88007cdd3b70>

I'm not sure of the full scenario of the reproduction, but my debug showed that
split_huge_pmd_address(freeze=true) returned without running main code of pmd
splitting because pmd_present(*pmd) was 0. If this happens, the subsequent
try_to_unmap() fails and returns non-zero (because page_mapcount() still > 0),
and finally VM_BUG_ON() fires.

This patch fixes it by adding a separate split_huge_pmd_address()'s variant
for freeze=true and checking pmd's state within ptl for that case.

I think that this change seems to fit the comment in split_huge_pmd_address()
that says "Caller holds the mmap_sem write mode, so a huge pmd cannot
materialize from under us." We don't hold mmap_sem write if called from
split_huge_page(), so maybe there were some different assumptions between
callers (split_huge_page() and vma_adjust_trans_huge().)

Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
---
 include/linux/huge_mm.h |  8 ++++----
 mm/huge_memory.c        | 50 +++++++++++++++++++++++++++++++++++++------------
 mm/rmap.c               |  3 +--
 3 files changed, 43 insertions(+), 18 deletions(-)

diff --git v4.6/include/linux/huge_mm.h v4.6_patched/include/linux/huge_mm.h
index d7b9e53..6fa4348 100644
--- v4.6/include/linux/huge_mm.h
+++ v4.6_patched/include/linux/huge_mm.h
@@ -108,8 +108,8 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	}  while (0)
 
 
-void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
-		bool freeze, struct page *page);
+void split_huge_pmd_address_freeze(struct vm_area_struct *vma,
+		unsigned long address, struct page *page);
 
 extern int hugepage_madvise(struct vm_area_struct *vma,
 			    unsigned long *vm_flags, int advice);
@@ -177,8 +177,8 @@ static inline void deferred_split_huge_page(struct page *page) {}
 #define split_huge_pmd(__vma, __pmd, __address)	\
 	do { } while (0)
 
-static inline void split_huge_pmd_address(struct vm_area_struct *vma,
-		unsigned long address, bool freeze, struct page *page) {}
+static inline void split_huge_pmd_address_freeze(struct vm_area_struct *vma,
+		unsigned long address, struct page *page) {}
 
 static inline int hugepage_madvise(struct vm_area_struct *vma,
 				   unsigned long *vm_flags, int advice)
diff --git v4.6/mm/huge_memory.c v4.6_patched/mm/huge_memory.c
index b49ee12..c48f22c 100644
--- v4.6/mm/huge_memory.c
+++ v4.6_patched/mm/huge_memory.c
@@ -2989,6 +2989,16 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 
 	mmu_notifier_invalidate_range_start(mm, haddr, haddr + HPAGE_PMD_SIZE);
 	ptl = pmd_lock(mm, pmd);
+	if (freeze) {
+		/*
+		 * If caller asks to setup a migration entries, we need a page
+		 * to check pmd against. Otherwise we can end up replacing
+		 * wrong page.
+		 */
+		VM_BUG_ON(freeze && !pmd_page(*pmd));
+		if (!pmd_present(*pmd))
+			goto out;
+	}
 	if (pmd_trans_huge(*pmd)) {
 		struct page *page = pmd_page(*pmd);
 		if (PageMlocked(page))
@@ -3001,8 +3011,8 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	mmu_notifier_invalidate_range_end(mm, haddr, haddr + HPAGE_PMD_SIZE);
 }
 
-void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
-		bool freeze, struct page *page)
+static void split_huge_pmd_address(struct vm_area_struct *vma,
+		unsigned long address, struct page *page)
 {
 	pgd_t *pgd;
 	pud_t *pud;
@@ -3019,12 +3029,6 @@ void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
 	pmd = pmd_offset(pud, address);
 	if (!pmd_present(*pmd) || (!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)))
 		return;
-
-	/*
-	 * If caller asks to setup a migration entries, we need a page to check
-	 * pmd against. Otherwise we can end up replacing wrong page.
-	 */
-	VM_BUG_ON(freeze && !page);
 	if (page && page != pmd_page(*pmd))
 		return;
 
@@ -3032,7 +3036,29 @@ void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
 	 * Caller holds the mmap_sem write mode, so a huge pmd cannot
 	 * materialize from under us.
 	 */
-	__split_huge_pmd(vma, pmd, address, freeze);
+	__split_huge_pmd(vma, pmd, address, false);
+}
+
+void split_huge_pmd_address_freeze(struct vm_area_struct *vma,
+				unsigned long address, struct page *page)
+{
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	pgd = pgd_offset(vma->vm_mm, address);
+	if (!pgd_present(*pgd))
+		return;
+
+	pud = pud_offset(pgd, address);
+	if (!pud_present(*pud))
+		return;
+
+	pmd = pmd_offset(pud, address);
+	if (pmd_none(*pmd))
+		return;
+
+	__split_huge_pmd(vma, pmd, address, true);
 }
 
 void vma_adjust_trans_huge(struct vm_area_struct *vma,
@@ -3048,7 +3074,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
 	if (start & ~HPAGE_PMD_MASK &&
 	    (start & HPAGE_PMD_MASK) >= vma->vm_start &&
 	    (start & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
-		split_huge_pmd_address(vma, start, false, NULL);
+		split_huge_pmd_address(vma, start, NULL);
 
 	/*
 	 * If the new end address isn't hpage aligned and it could
@@ -3058,7 +3084,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
 	if (end & ~HPAGE_PMD_MASK &&
 	    (end & HPAGE_PMD_MASK) >= vma->vm_start &&
 	    (end & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
-		split_huge_pmd_address(vma, end, false, NULL);
+		split_huge_pmd_address(vma, end, NULL);
 
 	/*
 	 * If we're also updating the vma->vm_next->vm_start, if the new
@@ -3072,7 +3098,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
 		if (nstart & ~HPAGE_PMD_MASK &&
 		    (nstart & HPAGE_PMD_MASK) >= next->vm_start &&
 		    (nstart & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= next->vm_end)
-			split_huge_pmd_address(next, nstart, false, NULL);
+			split_huge_pmd_address(next, nstart, NULL);
 	}
 }
 
diff --git v4.6/mm/rmap.c v4.6_patched/mm/rmap.c
index 307b555..4282b56 100644
--- v4.6/mm/rmap.c
+++ v4.6_patched/mm/rmap.c
@@ -1418,8 +1418,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 		goto out;
 
 	if (flags & TTU_SPLIT_HUGE_PMD) {
-		split_huge_pmd_address(vma, address,
-				flags & TTU_MIGRATION, page);
+		split_huge_pmd_address_freeze(vma, address, page);
 		/* check if we have anything to do after split */
 		if (page_mapcount(page) == 0)
 			goto out;
-- 
2.7.0

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