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Subject[PATCH v1 1/2] mm: thp: move pmd check inside ptl for freeze_page()
Date
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@ah.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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