Commit dd906184 authored by Boaz Harrosh's avatar Boaz Harrosh Committed by Linus Torvalds
Browse files

mm: new pfn_mkwrite same as page_mkwrite for VM_PFNMAP 



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.

Signed-off-by: default avatarYigal Korman <yigal@plexistor.com>
Signed-off-by: default avatarBoaz Harrosh <boaz@plexistor.com>
Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2682582a

Documentation/filesystems/Locking

+8 −0
Original line number Diff line number Diff line
@@ -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 returns

an error.



	->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

include/linux/mm.h

+3 −0
Original line number Diff line number Diff line
@@ -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

	 */

mm/memory.c

+39 −4
Original line number Diff line number Diff line
@@ -2180,6 +2180,42 @@ 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);

		/*

		 * We might have raced with another page fault while we

		 * released the pte_offset_map_lock.

		 */

		if (!pte_same(*page_table, orig_pte)) {

			pte_unmap_unlock(page_table, ptl);

			return 0;

		}

	}

	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,
@@ -2258,13 +2294,12 @@ 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,

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