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Commit 956a4cd2 authored by Dan Williams's avatar Dan Williams
Browse files

device-dax: switch to srcu, fix rcu_read_lock() vs pte allocation 



The following warning triggers with a new unit test that stresses the
device-dax interface.

 ===============================
 [ ERR: suspicious RCU usage.  ]
 4.11.0-rc4+ #1049 Tainted: G           O
 -------------------------------
 ./include/linux/rcupdate.h:521 Illegal context switch in RCU read-side critical section!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 0
 2 locks held by fio/9070:
  #0:  (&mm->mmap_sem){++++++}, at: [<ffffffff8d0739d7>] __do_page_fault+0x167/0x4f0
  #1:  (rcu_read_lock){......}, at: [<ffffffffc03fbd02>] dax_dev_huge_fault+0x32/0x620 [dax]

 Call Trace:
  dump_stack+0x86/0xc3
  lockdep_rcu_suspicious+0xd7/0x110
  ___might_sleep+0xac/0x250
  __might_sleep+0x4a/0x80
  __alloc_pages_nodemask+0x23a/0x360
  alloc_pages_current+0xa1/0x1f0
  pte_alloc_one+0x17/0x80
  __pte_alloc+0x1e/0x120
  __get_locked_pte+0x1bf/0x1d0
  insert_pfn.isra.70+0x3a/0x100
  ? lookup_memtype+0xa6/0xd0
  vm_insert_mixed+0x64/0x90
  dax_dev_huge_fault+0x520/0x620 [dax]
  ? dax_dev_huge_fault+0x32/0x620 [dax]
  dax_dev_fault+0x10/0x20 [dax]
  __do_fault+0x1e/0x140
  __handle_mm_fault+0x9af/0x10d0
  handle_mm_fault+0x16d/0x370
  ? handle_mm_fault+0x47/0x370
  __do_page_fault+0x28c/0x4f0
  trace_do_page_fault+0x58/0x2a0
  do_async_page_fault+0x1a/0xa0
  async_page_fault+0x28/0x30

Inserting a page table entry may trigger an allocation while we are
holding a read lock to keep the device instance alive for the duration
of the fault. Use srcu for this keep-alive protection.

Fixes: dee41079 ("/dev/dax, core: file operations and dax-mmap")
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
parent 4aa5615e

drivers/dax/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -2,6 +2,7 @@ menuconfig DEV_DAX 
	tristate "DAX: direct access to differentiated memory"

	default m if NVDIMM_DAX

	depends on TRANSPARENT_HUGEPAGE

	select SRCU

	help

	  Support raw access to differentiated (persistence, bandwidth,

	  latency...) memory via an mmap(2) capable character

drivers/dax/dax.c

+7 −6
Original line number Diff line number Diff line
@@ -25,6 +25,7 @@ 
#include "dax.h"



static dev_t dax_devt;

DEFINE_STATIC_SRCU(dax_srcu);

static struct class *dax_class;

static DEFINE_IDA(dax_minor_ida);

static int nr_dax = CONFIG_NR_DEV_DAX;
@@ -60,7 +61,7 @@ struct dax_region { 
 * @region - parent region

 * @dev - device backing the character device

 * @cdev - core chardev data

 * @alive - !alive + rcu grace period == no new mappings can be established

 * @alive - !alive + srcu grace period == no new mappings can be established

 * @id - child id in the region

 * @num_resources - number of physical address extents in this device

 * @res - array of physical address ranges
@@ -569,7 +570,7 @@ static int __dax_dev_pud_fault(struct dax_dev *dax_dev, struct vm_fault *vmf) 
static int dax_dev_huge_fault(struct vm_fault *vmf,

		enum page_entry_size pe_size)

{

	int rc;

	int rc, id;

	struct file *filp = vmf->vma->vm_file;

	struct dax_dev *dax_dev = filp->private_data;
@@ -578,7 +579,7 @@ static int dax_dev_huge_fault(struct vm_fault *vmf, 
			? "write" : "read",

			vmf->vma->vm_start, vmf->vma->vm_end);



	rcu_read_lock();

	id = srcu_read_lock(&dax_srcu);

	switch (pe_size) {

	case PE_SIZE_PTE:

		rc = __dax_dev_pte_fault(dax_dev, vmf);
@@ -592,7 +593,7 @@ static int dax_dev_huge_fault(struct vm_fault *vmf, 
	default:

		return VM_FAULT_FALLBACK;

	}

	rcu_read_unlock();

	srcu_read_unlock(&dax_srcu, id);



	return rc;

}
@@ -713,11 +714,11 @@ static void unregister_dax_dev(void *dev) 
	 * Note, rcu is not protecting the liveness of dax_dev, rcu is

	 * ensuring that any fault handlers that might have seen

	 * dax_dev->alive == true, have completed.  Any fault handlers

	 * that start after synchronize_rcu() has started will abort

	 * that start after synchronize_srcu() has started will abort

	 * upon seeing dax_dev->alive == false.

	 */

	dax_dev->alive = false;

	synchronize_rcu();

	synchronize_srcu(&dax_srcu);

	unmap_mapping_range(dax_dev->inode->i_mapping, 0, 0, 1);

	cdev_del(cdev);

	device_unregister(dev);