Re: [PATCH 10/22] memremap: add a migrate callback to struct dev_pagemap_ops

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On 6/13/19 2:43 AM, Christoph Hellwig wrote:
This replaces the hacky ->fault callback, which is currently directly
called from common code through a hmm specific data structure as an
exercise in layering violations.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
  include/linux/hmm.h      |  6 ------
  include/linux/memremap.h |  6 ++++++
  include/linux/swapops.h  | 15 ---------------
  kernel/memremap.c        | 31 -------------------------------
  mm/hmm.c                 | 13 +++++--------
  mm/memory.c              |  9 ++-------
  6 files changed, 13 insertions(+), 67 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 5761a39221a6..3c9a59dbfdb8 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -658,11 +658,6 @@ struct hmm_devmem_ops {
   * chunk, as an optimization. It must, however, prioritize the faulting address
   * over all the others.
   */
-typedef vm_fault_t (*dev_page_fault_t)(struct vm_area_struct *vma,
-				unsigned long addr,
-				const struct page *page,
-				unsigned int flags,
-				pmd_t *pmdp);
struct hmm_devmem {
  	struct completion		completion;
@@ -673,7 +668,6 @@ struct hmm_devmem {
  	struct dev_pagemap		pagemap;
  	const struct hmm_devmem_ops	*ops;
  	struct percpu_ref		ref;
-	dev_page_fault_t		page_fault;
  };
/*
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 96a3a6d564ad..03a4099be701 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -75,6 +75,12 @@ struct dev_pagemap_ops {
  	 * Transition the percpu_ref in struct dev_pagemap to the dead state.
  	 */
  	void (*kill)(struct dev_pagemap *pgmap);
+
+	/*
+	 * Used for private (un-addressable) device memory only.  Must migrate
+	 * the page back to a CPU accessible page.
+	 */
+	vm_fault_t (*migrate)(struct vm_fault *vmf);
  };
/**
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 4d961668e5fc..15bdb6fe71e5 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -129,12 +129,6 @@ static inline struct page *device_private_entry_to_page(swp_entry_t entry)
  {
  	return pfn_to_page(swp_offset(entry));
  }
-
-vm_fault_t device_private_entry_fault(struct vm_area_struct *vma,
-		       unsigned long addr,
-		       swp_entry_t entry,
-		       unsigned int flags,
-		       pmd_t *pmdp);
  #else /* CONFIG_DEVICE_PRIVATE */
  static inline swp_entry_t make_device_private_entry(struct page *page, bool write)
  {
@@ -164,15 +158,6 @@ static inline struct page *device_private_entry_to_page(swp_entry_t entry)
  {
  	return NULL;
  }
-
-static inline vm_fault_t device_private_entry_fault(struct vm_area_struct *vma,
-				     unsigned long addr,
-				     swp_entry_t entry,
-				     unsigned int flags,
-				     pmd_t *pmdp)
-{
-	return VM_FAULT_SIGBUS;
-}
  #endif /* CONFIG_DEVICE_PRIVATE */
#ifdef CONFIG_MIGRATION
diff --git a/kernel/memremap.c b/kernel/memremap.c
index 6a3183cac764..7167e717647d 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -11,7 +11,6 @@
  #include <linux/types.h>
  #include <linux/wait_bit.h>
  #include <linux/xarray.h>
-#include <linux/hmm.h>
static DEFINE_XARRAY(pgmap_array);
  #define SECTION_MASK ~((1UL << PA_SECTION_SHIFT) - 1)
@@ -48,36 +47,6 @@ static inline int dev_pagemap_enable(struct device *dev)
  }
  #endif /* CONFIG_DEV_PAGEMAP_OPS */
-#if IS_ENABLED(CONFIG_DEVICE_PRIVATE)
-vm_fault_t device_private_entry_fault(struct vm_area_struct *vma,
-		       unsigned long addr,
-		       swp_entry_t entry,
-		       unsigned int flags,
-		       pmd_t *pmdp)
-{
-	struct page *page = device_private_entry_to_page(entry);
-	struct hmm_devmem *devmem;
-
-	devmem = container_of(page->pgmap, typeof(*devmem), pagemap);
-
-	/*
-	 * The page_fault() callback must migrate page back to system memory
-	 * so that CPU can access it. This might fail for various reasons
-	 * (device issue, device was unsafely unplugged, ...). When such
-	 * error conditions happen, the callback must return VM_FAULT_SIGBUS.
-	 *
-	 * Note that because memory cgroup charges are accounted to the device
-	 * memory, this should never fail because of memory restrictions (but
-	 * allocation of regular system page might still fail because we are
-	 * out of memory).
-	 *
-	 * There is a more in-depth description of what that callback can and
-	 * cannot do, in include/linux/memremap.h
-	 */
-	return devmem->page_fault(vma, addr, page, flags, pmdp);
-}
-#endif /* CONFIG_DEVICE_PRIVATE */
-
  static void pgmap_array_delete(struct resource *res)
  {
  	xa_store_range(&pgmap_array, PHYS_PFN(res->start), PHYS_PFN(res->end),
diff --git a/mm/hmm.c b/mm/hmm.c
index 6dc769feb2e1..aab799677c7d 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -1330,15 +1330,12 @@ static void hmm_devmem_ref_kill(struct dev_pagemap *pgmap)
  	percpu_ref_kill(pgmap->ref);
  }
-static vm_fault_t hmm_devmem_fault(struct vm_area_struct *vma,
-			    unsigned long addr,
-			    const struct page *page,
-			    unsigned int flags,
-			    pmd_t *pmdp)
+static vm_fault_t hmm_devmem_migrate(struct vm_fault *vmf)
  {
-	struct hmm_devmem *devmem = page->pgmap->data;
+	struct hmm_devmem *devmem = vmf->page->pgmap->data;
- return devmem->ops->fault(devmem, vma, addr, page, flags, pmdp);
+	return devmem->ops->fault(devmem, vmf->vma, vmf->address, vmf->page,
+			vmf->flags, vmf->pmd);
  }
static void hmm_devmem_free(struct page *page, void *data)
@@ -1351,6 +1348,7 @@ static void hmm_devmem_free(struct page *page, void *data)
  static const struct dev_pagemap_ops hmm_pagemap_ops = {
  	.page_free		= hmm_devmem_free,
  	.kill			= hmm_devmem_ref_kill,
+	.migrate		= hmm_devmem_migrate,
  };
/*
@@ -1405,7 +1403,6 @@ struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
  	devmem->pfn_first = devmem->resource->start >> PAGE_SHIFT;
  	devmem->pfn_last = devmem->pfn_first +
  			   (resource_size(devmem->resource) >> PAGE_SHIFT);
-	devmem->page_fault = hmm_devmem_fault;
devmem->pagemap.type = MEMORY_DEVICE_PRIVATE;
  	devmem->pagemap.res = *devmem->resource;
diff --git a/mm/memory.c b/mm/memory.c
index ddf20bd0c317..cbf3cb598436 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2782,13 +2782,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
  			migration_entry_wait(vma->vm_mm, vmf->pmd,
  					     vmf->address);
  		} else if (is_device_private_entry(entry)) {
-			/*
-			 * For un-addressable device memory we call the pgmap
-			 * fault handler callback. The callback must migrate
-			 * the page back to some CPU accessible page.
-			 */
-			ret = device_private_entry_fault(vma, vmf->address, entry,
-						 vmf->flags, vmf->pmd);
+			vmf->page = device_private_entry_to_page(entry);
+			ret = page->pgmap->ops->migrate(vmf);

This needs to either initialize "page" or be changed to "vmf->page".
Otherwise, it is a NULL pointer dereference.

  		} else if (is_hwpoison_entry(entry)) {
  			ret = VM_FAULT_HWPOISON;
  		} else {


You can add:
Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
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