Re: [PATCH 4/8] mm: FOLL_DUMP replace FOLL_ANON

Mel Gorman
2009-09-09T11:15:22Z

On Mon, Sep 07, 2009 at 10:35:32PM +0100, Hugh Dickins wrote:
> 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: Hugh Dickins <hugh.dickins@tiscali.co.uk>

Acked-by: Mel Gorman <mel@csn.ul.ie>

> ---
>
> include/linux/mm.h | 2 +-
> mm/internal.h | 1 +
> mm/memory.c | 43 ++++++++++++-------------------------------
> 3 files changed, 14 insertions(+), 32 deletions(-)
>
> --- mm3/include/linux/mm.h 2009-09-07 13:16:32.000000000 +0100
> +++ mm4/include/linux/mm.h 2009-09-07 13:16:39.000000000 +0100
> @@ -1247,7 +1247,7 @@ struct page *follow_page(struct vm_area_
> #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);
> --- mm3/mm/internal.h 2009-09-07 13:16:22.000000000 +0100
> +++ mm4/mm/internal.h 2009-09-07 13:16:39.000000000 +0100
> @@ -252,6 +252,7 @@ static inline void mminit_validate_memmo
>
> #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,
> --- mm3/mm/memory.c 2009-09-07 13:16:32.000000000 +0100
> +++ mm4/mm/memory.c 2009-09-07 13:16:39.000000000 +0100
> @@ -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.
> + * 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_ANON) {
> - page = ZERO_PAGE(0);
> - if (flags & FOLL_GET)
> - get_page(page);
> - BUG_ON(flags & FOLL_WRITE);
> - }
> + 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
> 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;
> @@ -1447,7 +1428,7 @@ struct page *get_dump_page(unsigned long
> 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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>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab