On Tue, Apr 07, 2015 at 03:57:32PM +0300, Boaz Harrosh wrote:
>
> [v4]
> Kirill's comments about splitting out a new wp_pfn_shared().
> Add Documentation/filesystems/Locking text about pfn_mkwrite.
>
> [v3]
> Kirill's comments about use of linear_page_index()
>
> [v2]
> Based on linux-next/akpm [3dc4623]. For v4.1 merge window
> Incorporated comments from Andrew And Kirill
>
> [v1]
> This will allow FS that uses VM_PFNMAP | VM_MIXEDMAP (no page structs)
> to get notified when access is a write to a read-only PFN.
>
> This can happen if we mmap() a file then first mmap-read from it
> to page-in a read-only PFN, than we mmap-write to the same page.
>
> We need this functionality to fix a DAX bug, where in the scenario
> above we fail to set ctime/mtime though we modified the file.
> An xfstest is attached to this patchset that shows the failure
> and the fix. (A DAX patch will follow)
>
> This functionality is extra important for us, because upon
> dirtying of a pmem page we also want to RDMA the page to a
> remote cluster node.
>
> We define a new pfn_mkwrite and do not reuse page_mkwrite because
> 1 - The name ;-)
> 2 - But mainly because it would take a very long and tedious
> audit of all page_mkwrite functions of VM_MIXEDMAP/VM_PFNMAP
> users. To make sure they do not now CRASH. For example current
> DAX code (which this is for) would crash.
> If we would want to reuse page_mkwrite, We will need to first
> patch all users, so to not-crash-on-no-page. Then enable this
> patch. But even if I did that I would not sleep so well at night.
> Adding a new vector is the safest thing to do, and is not that
> expensive. an extra pointer at a static function vector per driver.
> Also the new vector is better for performance, because else we
> Will call all current Kernel vectors, so to:
> check-ha-no-page-do-nothing and return.
>
> No need to call it from do_shared_fault because do_wp_page is called to
> change pte permissions anyway.
>
> CC: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> CC: Matthew Wilcox <matthew.r.wilcox@xxxxxxxxx>
> CC: Jan Kara <jack@xxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Hugh Dickins <hughd@xxxxxxxxxx>
> CC: Mel Gorman <mgorman@xxxxxxx>
> CC: linux-mm@xxxxxxxxx
>
> Signed-off-by: Yigal Korman <yigal@xxxxxxxxxxxxx>
> Signed-off-by: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
> ---
> Documentation/filesystems/Locking | 8 ++++++++
> include/linux/mm.h | 3 +++
> mm/memory.c | 40 +++++++++++++++++++++++++++++++++++----
> 3 files changed, 47 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
> index f91926f..25f36e6 100644
> --- a/Documentation/filesystems/Locking
> +++ b/Documentation/filesystems/Locking
> @@ -525,6 +525,7 @@ prototypes:
> void (*close)(struct vm_area_struct*);
> int (*fault)(struct vm_area_struct*, struct vm_fault *);
> int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
> + int (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
> int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
>
> locking rules:
> @@ -534,6 +535,7 @@ close: yes
> fault: yes can return with page locked
> map_pages: yes
> page_mkwrite: yes can return with page locked
> +pfn_mkwrite: yes
> access: yes
>
> ->fault() is called when a previously not present pte is about
> @@ -560,6 +562,12 @@ the page has been truncated, the filesystem should not look up a new page
> like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
> will cause the VM to retry the fault.
>
> + ->pfn_mkwrite() is the same as page_mkwrite but when the pte is
> +VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
> +VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
> +after this call is to make the pte read-write, unless pfn_mkwrite()
> +already touched the pte, in that case it is untouched.
> +
> ->access() is called when get_user_pages() fails in
> access_process_vm(), typically used to debug a process through
> /proc/pid/mem or ptrace. This function is needed only for
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d584b95..70c47f2 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -251,6 +251,9 @@ struct vm_operations_struct {
> * writable, if an error is returned it will cause a SIGBUS */
> int (*page_mkwrite)(struct vm_area_struct *vma, struct vm_fault *vmf);
>
> + /* same as page_mkwrite when using VM_PFNMAP|VM_MIXEDMAP */
> + int (*pfn_mkwrite)(struct vm_area_struct *vma, struct vm_fault *vmf);
> +
> /* called by access_process_vm when get_user_pages() fails, typically
> * for use by special VMAs that can switch between memory and hardware
> */
> diff --git a/mm/memory.c b/mm/memory.c
> index 59f6268..67b1ee8 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2181,6 +2181,39 @@ oom:
> return VM_FAULT_OOM;
> }
>
> +/*
> + * Handle write page faults for VM_MIXEDMAP or VM_PFNMAP for a VM_SHARED
> + * mapping
> + */
> +static int wp_pfn_shared(struct mm_struct *mm,
> + struct vm_area_struct *vma, unsigned long address,
> + pte_t *page_table, spinlock_t *ptl, pte_t orig_pte,
> + pmd_t *pmd)
> +{
> + if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
> + struct vm_fault vmf = {
> + .page = NULL,
> + .pgoff = linear_page_index(vma, address),
> + .virtual_address = (void __user *)(address & PAGE_MASK),
> + .flags = FAULT_FLAG_WRITE | FAULT_FLAG_MKWRITE,
> + };
> + int ret;
> +
> + pte_unmap_unlock(page_table, ptl);
> + ret = vma->vm_ops->pfn_mkwrite(vma, &vmf);
> + if (ret & VM_FAULT_ERROR)
> + return ret;
> + page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
> + /* Did pfn_mkwrite already fixed up the pte */
> + if (!pte_same(*page_table, orig_pte)) {
> + pte_unmap_unlock(page_table, ptl);
> + return ret;
This should be "return 0;", shouldn't it?
VM_FAULT_NOPAGE would imply you've installed new pte, but you did not.
> + }
> + }
> + return wp_page_reuse(mm, vma, address, page_table, ptl, orig_pte,
> + NULL, 0, 0);
> +}
> +
> static int wp_page_shared(struct mm_struct *mm, struct vm_area_struct *vma,
> unsigned long address, pte_t *page_table,
> pmd_t *pmd, spinlock_t *ptl, pte_t orig_pte,
> @@ -2259,13 +2292,12 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> * VM_PFNMAP VMA.
> *
> * We should not cow pages in a shared writeable mapping.
> - * Just mark the pages writable as we can't do any dirty
> - * accounting on raw pfn maps.
> + * Just mark the pages writable and/or call ops->pfn_mkwrite.
> */
> if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
> (VM_WRITE|VM_SHARED))
> - return wp_page_reuse(mm, vma, address, page_table, ptl,
> - orig_pte, old_page, 0, 0);
> + return wp_pfn_shared(mm, vma, address, page_table, ptl,
> + orig_pte, pmd);
>
> pte_unmap_unlock(page_table, ptl);
> return wp_page_copy(mm, vma, address, page_table, pmd,
> --
> 1.9.3
>
>
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--
Kirill A. Shutemov
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