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

Ralph Campbell
2019-06-13T23:42:38Z

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@lst.de>
> ---
> 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@nvidia.com>