On Sun, Feb 03, 2019 at 09:21:33PM -0800, john.hubbard@xxxxxxxxx wrote:
> From: John Hubbard <jhubbard@xxxxxxxxxx>
>
[snip]
>
> +/*
> + * GUP_PIN_COUNTING_BIAS, and the associated functions that use it, overload
> + * the page's refcount so that two separate items are tracked: the original page
> + * reference count, and also a new count of how many get_user_pages() calls were
> + * made against the page. ("gup-pinned" is another term for the latter).
> + *
> + * With this scheme, get_user_pages() becomes special: such pages are marked
> + * as distinct from normal pages. As such, the new put_user_page() call (and
> + * its variants) must be used in order to release gup-pinned pages.
> + *
> + * Choice of value:
> + *
> + * By making GUP_PIN_COUNTING_BIAS a power of two, debugging of page reference
> + * counts with respect to get_user_pages() and put_user_page() becomes simpler,
> + * due to the fact that adding an even power of two to the page refcount has
> + * the effect of using only the upper N bits, for the code that counts up using
> + * the bias value. This means that the lower bits are left for the exclusive
> + * use of the original code that increments and decrements by one (or at least,
> + * by much smaller values than the bias value).
> + *
> + * Of course, once the lower bits overflow into the upper bits (and this is
> + * OK, because subtraction recovers the original values), then visual inspection
> + * no longer suffices to directly view the separate counts. However, for normal
> + * applications that don't have huge page reference counts, this won't be an
> + * issue.
> + *
> + * This has to work on 32-bit as well as 64-bit systems. In the more constrained
> + * 32-bit systems, the 10 bit value of the bias value leaves 22 bits for the
> + * upper bits. Therefore, only about 4M calls to get_user_page() may occur for
> + * a page.
> + *
> + * Locking: the lockless algorithm described in page_cache_gup_pin_speculative()
> + * and page_cache_gup_pin_speculative() provides safe operation for
Did you mean:
page_cache_gup_pin_speculative and __ page_cache_get_speculative __?
Just found this while looking at your branch.
Sorry,
Ira
> + * get_user_pages and page_mkclean and other calls that race to set up page
> + * table entries.
> + */
> +#define GUP_PIN_COUNTING_BIAS (1UL << 10)
> +
> +int get_gup_pin_page(struct page *page);
> +
> +void put_user_page(struct page *page);
> +void put_user_pages_dirty(struct page **pages, unsigned long npages);
> +void put_user_pages_dirty_lock(struct page **pages, unsigned long npages);
> +void put_user_pages(struct page **pages, unsigned long npages);
> +
> +/**
> + * page_gup_pinned() - report if a page is gup-pinned (pinned by a call to
> + * get_user_pages).
> + * @page: pointer to page to be queried.
> + * @Returns: True, if it is likely that the page has been "gup-pinned".
> + * False, if the page is definitely not gup-pinned.
> + */
> +static inline bool page_gup_pinned(struct page *page)
> +{
> + return (page_ref_count(page)) > GUP_PIN_COUNTING_BIAS;
> +}
> +
> static inline void get_page(struct page *page)
> {
> page = compound_head(page);
> @@ -993,30 +1050,6 @@ static inline void put_page(struct page *page)
> __put_page(page);
> }
>
> -/**
> - * put_user_page() - release a gup-pinned page
> - * @page: pointer to page to be released
> - *
> - * Pages that were pinned via get_user_pages*() must be released via
> - * either put_user_page(), or one of the put_user_pages*() routines
> - * below. This is so that eventually, pages that are pinned via
> - * get_user_pages*() can be separately tracked and uniquely handled. In
> - * particular, interactions with RDMA and filesystems need special
> - * handling.
> - *
> - * put_user_page() and put_page() are not interchangeable, despite this early
> - * implementation that makes them look the same. put_user_page() calls must
> - * be perfectly matched up with get_user_page() calls.
> - */
> -static inline void put_user_page(struct page *page)
> -{
> - put_page(page);
> -}
> -
> -void put_user_pages_dirty(struct page **pages, unsigned long npages);
> -void put_user_pages_dirty_lock(struct page **pages, unsigned long npages);
> -void put_user_pages(struct page **pages, unsigned long npages);
> -
> #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> #define SECTION_IN_PAGE_FLAGS
> #endif
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 5c8a9b59cbdc..5f5b72ba595f 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -209,6 +209,11 @@ static inline int page_cache_add_speculative(struct page *page, int count)
> return __page_cache_add_speculative(page, count);
> }
>
> +static inline int page_cache_gup_pin_speculative(struct page *page)
> +{
> + return __page_cache_add_speculative(page, GUP_PIN_COUNTING_BIAS);
> +}
> +
> #ifdef CONFIG_NUMA
> extern struct page *__page_cache_alloc(gfp_t gfp);
> #else
> diff --git a/mm/gup.c b/mm/gup.c
> index 05acd7e2eb22..3291da342f9c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -25,6 +25,26 @@ struct follow_page_context {
> unsigned int page_mask;
> };
>
> +/**
> + * get_gup_pin_page() - mark a page as being used by get_user_pages().
> + * @page: pointer to page to be marked
> + * @Returns: 0 for success, -EOVERFLOW if the page refcount would have
> + * overflowed.
> + *
> + */
> +int get_gup_pin_page(struct page *page)
> +{
> + page = compound_head(page);
> +
> + if (page_ref_count(page) >= (UINT_MAX - GUP_PIN_COUNTING_BIAS)) {
> + WARN_ONCE(1, "get_user_pages pin count overflowed");
> + return -EOVERFLOW;
> + }
> +
> + page_ref_add(page, GUP_PIN_COUNTING_BIAS);
> + return 0;
> +}
> +
> static struct page *no_page_table(struct vm_area_struct *vma,
> unsigned int flags)
> {
> @@ -157,8 +177,14 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> goto retry;
> }
>
> - if (flags & FOLL_GET)
> - get_page(page);
> + if (flags & FOLL_GET) {
> + int ret = get_gup_pin_page(page);
> +
> + if (ret) {
> + page = ERR_PTR(ret);
> + goto out;
> + }
> + }
> if (flags & FOLL_TOUCH) {
> if ((flags & FOLL_WRITE) &&
> !pte_dirty(pte) && !PageDirty(page))
> @@ -497,7 +523,10 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
> if (is_device_public_page(*page))
> goto unmap;
> }
> - get_page(*page);
> +
> + ret = get_gup_pin_page(*page);
> + if (ret)
> + goto unmap;
> out:
> ret = 0;
> unmap:
> @@ -1429,11 +1458,11 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> page = pte_page(pte);
> head = compound_head(page);
>
> - if (!page_cache_get_speculative(head))
> + if (!page_cache_gup_pin_speculative(head))
> goto pte_unmap;
>
> if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> - put_page(head);
> + put_user_page(head);
> goto pte_unmap;
> }
>
> @@ -1488,7 +1517,11 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
> }
> SetPageReferenced(page);
> pages[*nr] = page;
> - get_page(page);
> + if (get_gup_pin_page(page)) {
> + undo_dev_pagemap(nr, nr_start, pages);
> + return 0;
> + }
> +
> (*nr)++;
> pfn++;
> } while (addr += PAGE_SIZE, addr != end);
> @@ -1569,15 +1602,14 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> } while (addr += PAGE_SIZE, addr != end);
>
> head = compound_head(pmd_page(orig));
> - if (!page_cache_add_speculative(head, refs)) {
> + if (!page_cache_gup_pin_speculative(head)) {
> *nr -= refs;
> return 0;
> }
>
> if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
> *nr -= refs;
> - while (refs--)
> - put_page(head);
> + put_user_page(head);
> return 0;
> }
>
> @@ -1607,15 +1639,14 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> } while (addr += PAGE_SIZE, addr != end);
>
> head = compound_head(pud_page(orig));
> - if (!page_cache_add_speculative(head, refs)) {
> + if (!page_cache_gup_pin_speculative(head)) {
> *nr -= refs;
> return 0;
> }
>
> if (unlikely(pud_val(orig) != pud_val(*pudp))) {
> *nr -= refs;
> - while (refs--)
> - put_page(head);
> + put_user_page(head);
> return 0;
> }
>
> @@ -1644,15 +1675,14 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
> } while (addr += PAGE_SIZE, addr != end);
>
> head = compound_head(pgd_page(orig));
> - if (!page_cache_add_speculative(head, refs)) {
> + if (!page_cache_gup_pin_speculative(head)) {
> *nr -= refs;
> return 0;
> }
>
> if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
> *nr -= refs;
> - while (refs--)
> - put_page(head);
> + put_user_page(head);
> return 0;
> }
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 7c42ca45bb89..39b0ddd35933 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -133,6 +133,27 @@ void put_pages_list(struct list_head *pages)
> }
> EXPORT_SYMBOL(put_pages_list);
>
> +/**
> + * put_user_page() - release a gup-pinned page
> + * @page: pointer to page to be released
> + *
> + * Pages that were pinned via get_user_pages*() must be released via
> + * either put_user_page(), or one of the put_user_pages*() routines
> + * below. This is so that eventually, pages that are pinned via
> + * get_user_pages*() can be separately tracked and uniquely handled. In
> + * particular, interactions with RDMA and filesystems need special
> + * handling.
> + */
> +void put_user_page(struct page *page)
> +{
> + page = compound_head(page);
> +
> + VM_BUG_ON_PAGE(page_ref_count(page) < GUP_PIN_COUNTING_BIAS, page);
> +
> + page_ref_sub(page, GUP_PIN_COUNTING_BIAS);
> +}
> +EXPORT_SYMBOL(put_user_page);
> +
> typedef int (*set_dirty_func)(struct page *page);
>
> static void __put_user_pages_dirty(struct page **pages,
> --
> 2.20.1
>
