Commit 8e4b9a60 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

mm: FOLL_DUMP replace FOLL_ANON 



The "FOLL_ANON optimization" and its use_zero_page() test have caused
confusion and bugs: why does it test VM_SHARED? for the very good but
unsatisfying reason that VMware crashed without.  As we look to maybe
reinstating anonymous use of the ZERO_PAGE, we need to sort this out.

Easily done: it's silly for __get_user_pages() and follow_page() to
be guessing whether it's safe to assume that they're being used for
a coredump (which can take a shortcut snapshot where other uses must
handle a fault) - just tell them with GUP_FLAGS_DUMP and FOLL_DUMP.

get_dump_page() doesn't even want a ZERO_PAGE: an error suits fine.

Signed-off-by: default avatarHugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: default avatarRik van Riel <riel@redhat.com>
Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
Reviewed-by: default avatarMinchan Kim <minchan.kim@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f3e8fccd

include/linux/mm.h

+1 −1
Original line number Diff line number Diff line
@@ -1231,7 +1231,7 @@ struct page *follow_page(struct vm_area_struct *, unsigned long address, 
#define FOLL_WRITE	0x01	/* check pte is writable */

#define FOLL_TOUCH	0x02	/* mark page accessed */

#define FOLL_GET	0x04	/* do get_page on page */

#define FOLL_ANON	0x08	/* give ZERO_PAGE if no pgtable */

#define FOLL_DUMP	0x08	/* give error on hole if it would be zero */



typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,

			void *data);

mm/internal.h

+1 −0
Original line number Diff line number Diff line
@@ -252,6 +252,7 @@ static inline void mminit_validate_memmodel_limits(unsigned long *start_pfn, 


#define GUP_FLAGS_WRITE		0x01

#define GUP_FLAGS_FORCE		0x02

#define GUP_FLAGS_DUMP		0x04



int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,

		     unsigned long start, int len, int flags,

mm/memory.c

+12 −31
Original line number Diff line number Diff line
@@ -1174,41 +1174,22 @@ no_page: 
	pte_unmap_unlock(ptep, ptl);

	if (!pte_none(pte))

		return page;

	/* Fall through to ZERO_PAGE handling */



no_page_table:

	/*

	 * When core dumping an enormous anonymous area that nobody

	 * has touched so far, we don't want to allocate page tables.

	 */

	if (flags & FOLL_ANON) {

		page = ZERO_PAGE(0);

		if (flags & FOLL_GET)

			get_page(page);

		BUG_ON(flags & FOLL_WRITE);

	}

	 * has touched so far, we don't want to allocate unnecessary pages or

	 * page tables.  Return error instead of NULL to skip handle_mm_fault,

	 * then get_dump_page() will return NULL to leave a hole in the dump.

	 * But we can only make this optimization where a hole would surely

	 * be zero-filled if handle_mm_fault() actually did handle it.

	 */

	if ((flags & FOLL_DUMP) &&

	    (!vma->vm_ops || !vma->vm_ops->fault))

		return ERR_PTR(-EFAULT);

	return page;

}



/* Can we do the FOLL_ANON optimization? */

static inline int use_zero_page(struct vm_area_struct *vma)

{

	/*

	 * We don't want to optimize FOLL_ANON for make_pages_present()

	 * when it tries to page in a VM_LOCKED region. As to VM_SHARED,

	 * we want to get the page from the page tables to make sure

	 * that we serialize and update with any other user of that

	 * mapping.

	 */

	if (vma->vm_flags & (VM_LOCKED | VM_SHARED))

		return 0;

	/*

	 * And if we have a fault routine, it's not an anonymous region.

	 */

	return !vma->vm_ops || !vma->vm_ops->fault;

}







int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,

		     unsigned long start, int nr_pages, int flags,

		     struct page **pages, struct vm_area_struct **vmas)
@@ -1288,8 +1269,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, 
		foll_flags = FOLL_TOUCH;

		if (pages)

			foll_flags |= FOLL_GET;

		if (!write && use_zero_page(vma))

			foll_flags |= FOLL_ANON;

		if (flags & GUP_FLAGS_DUMP)

			foll_flags |= FOLL_DUMP;



		do {

			struct page *page;
@@ -1446,7 +1427,7 @@ struct page *get_dump_page(unsigned long addr) 
	struct page *page;



	if (__get_user_pages(current, current->mm, addr, 1,

				GUP_FLAGS_FORCE, &page, &vma) < 1)

			GUP_FLAGS_FORCE | GUP_FLAGS_DUMP, &page, &vma) < 1)

		return NULL;

	if (page == ZERO_PAGE(0)) {

		page_cache_release(page);

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