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SubjectRe: [PATCH 6/6] mm: track gup pages with page->dma_pinned_* fields
On Thu, Oct 11, 2018 at 11:00:14PM -0700, john.hubbard@gmail.com wrote:
> From: John Hubbard <jhubbard@nvidia.com>
>
> This patch sets and restores the new page->dma_pinned_flags and
> page->dma_pinned_count fields, but does not actually use them for
> anything yet.
>
> In order to use these fields at all, the page must be removed from
> any LRU list that it's on. The patch also adds some precautions that
> prevent the page from getting moved back onto an LRU, once it is
> in this state.
>
> This is in preparation to fix some problems that came up when using
> devices (NICs, GPUs, for example) that set up direct access to a chunk
> of system (CPU) memory, so that they can DMA to/from that memory.
>
> CC: Matthew Wilcox <willy@infradead.org>
> CC: Jan Kara <jack@suse.cz>
> CC: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
> include/linux/mm.h | 15 ++-----------
> mm/gup.c | 56 ++++++++++++++++++++++++++++++++++++++++++++--
> mm/memcontrol.c | 7 ++++++
> mm/swap.c | 51 +++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 114 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 76d18aada9f8..44878d21e27b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -944,21 +944,10 @@ static inline void put_page(struct page *page)
> }
>
> /*
> - * put_user_page() - release a page that had previously been acquired via
> - * a call to one of the get_user_pages*() functions.
> - *
> * 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.
> + * one of these put_user_pages*() routines:
> */
> -static inline void put_user_page(struct page *page)
> -{
> - put_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);
> diff --git a/mm/gup.c b/mm/gup.c
> index 05ee7c18e59a..fddbc30cde89 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -20,6 +20,51 @@
>
> #include "internal.h"
>
> +static int pin_page_for_dma(struct page *page)
> +{
> + int ret = 0;
> + struct zone *zone;
> +
> + page = compound_head(page);
> + zone = page_zone(page);
> +
> + spin_lock(zone_gup_lock(zone));
> +
> + if (PageDmaPinned(page)) {
> + /* Page was not on an LRU list, because it was DMA-pinned. */
> + VM_BUG_ON_PAGE(PageLRU(page), page);
> +
> + atomic_inc(&page->dma_pinned_count);
> + goto unlock_out;
> + }
> +
> + /*
> + * Note that page->dma_pinned_flags is unioned with page->lru.
> + * Therefore, the rules are: checking if any of the
> + * PAGE_DMA_PINNED_FLAGS bits are set may be done while page->lru
> + * is in use. However, setting those flags requires that
> + * the page is both locked, and also, removed from the LRU.
> + */
> + ret = isolate_lru_page(page);
> +

isolate_lru_page() can be expensive and in terms of the overall locking order
sounds like zone_gup_lock is higher in the hierarchy than the locks taken
inside isolate_lru_page()

> + if (ret == 0) {
> + /* Avoid problems later, when freeing the page: */
> + ClearPageActive(page);
> + ClearPageUnevictable(page);
> +
> + /* counteract isolate_lru_page's effects: */
> + put_page(page);

Can the page get reaped here? What's the expected page count?

> +
> + atomic_set(&page->dma_pinned_count, 1);
> + SetPageDmaPinned(page);
> + }
> +
> +unlock_out:
> + spin_unlock(zone_gup_lock(zone));
> +
> + return ret;
> +}
> +
> static struct page *no_page_table(struct vm_area_struct *vma,
> unsigned int flags)
> {
> @@ -659,7 +704,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> unsigned int gup_flags, struct page **pages,
> struct vm_area_struct **vmas, int *nonblocking)
> {
> - long i = 0;
> + long i = 0, j;
> int err = 0;
> unsigned int page_mask;
> struct vm_area_struct *vma = NULL;
> @@ -764,6 +809,10 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> } while (nr_pages);
>
> out:
> + if (pages)
> + for (j = 0; j < i; j++)
> + pin_page_for_dma(pages[j]);
> +

Why does get_user_pages() unconditionally pin_page_for_dma?

> return i ? i : err;
> }
>
> @@ -1841,7 +1890,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
> struct page **pages)
> {
> unsigned long addr, len, end;
> - int nr = 0, ret = 0;
> + int nr = 0, ret = 0, i;
>
> start &= PAGE_MASK;
> addr = start;
> @@ -1862,6 +1911,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
> ret = nr;
> }
>
> + for (i = 0; i < nr; i++)
> + pin_page_for_dma(pages[i]);

Why does get_user_pages_fast() unconditionally pin_page_for_dma?

> +
> if (nr < nr_pages) {
> /* Try to get the remaining pages with get_user_pages */
> start += nr << PAGE_SHIFT;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e79cb59552d9..af9719756081 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2335,6 +2335,11 @@ static void lock_page_lru(struct page *page, int *isolated)
> if (PageLRU(page)) {
> struct lruvec *lruvec;
>
> + /* LRU and PageDmaPinned are mutually exclusive: they use the
> + * same fields in struct page, but for different purposes.
> + */

Comment style needs fixing

> + VM_BUG_ON_PAGE(PageDmaPinned(page), page);
> +
> lruvec = mem_cgroup_page_lruvec(page, zone->zone_pgdat);
> ClearPageLRU(page);
> del_page_from_lru_list(page, lruvec, page_lru(page));
> @@ -2352,6 +2357,8 @@ static void unlock_page_lru(struct page *page, int isolated)
>
> lruvec = mem_cgroup_page_lruvec(page, zone->zone_pgdat);
> VM_BUG_ON_PAGE(PageLRU(page), page);
> + VM_BUG_ON_PAGE(PageDmaPinned(page), page);
> +
> SetPageLRU(page);
> add_page_to_lru_list(page, lruvec, page_lru(page));
> }
> diff --git a/mm/swap.c b/mm/swap.c
> index efab3a6b6f91..6b2b1a958a67 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -134,6 +134,46 @@ void put_pages_list(struct list_head *pages)
> }
> EXPORT_SYMBOL(put_pages_list);
>
> +/*
> + * put_user_page() - release a page that had previously been acquired via
> + * a call to one of the get_user_pages*() functions.
> + *
> + * 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)
> +{
> + struct zone *zone = page_zone(page);
> +
> + page = compound_head(page);
> +
> + VM_BUG_ON_PAGE(PageLRU(page), page);
> + VM_BUG_ON_PAGE(!PageDmaPinned(page), page);
> +
> + if (atomic_dec_and_test(&page->dma_pinned_count)) {
> + spin_lock(zone_gup_lock(zone));
> +
> + /* Re-check while holding the lock, because
> + * pin_page_for_dma() or get_page() may have snuck in right
> + * after the atomic_dec_and_test, and raised the count
> + * above zero again. If so, just leave the flag set. And
> + * because the atomic_dec_and_test above already got the
> + * accounting correct, no other action is required.
> + */
> + if (atomic_read(&page->dma_pinned_count) == 0)
> + ClearPageDmaPinned(page);
> +
> + spin_unlock(zone_gup_lock(zone));
> + }
> +
> + put_page(page);
> +}
> +EXPORT_SYMBOL(put_user_page);
> +
> /*
> * put_user_pages_dirty() - for each page in the @pages array, make
> * that page (or its head page, if a compound page) dirty, if it was
> @@ -907,6 +947,11 @@ void lru_add_page_tail(struct page *page, struct page *page_tail,
> VM_BUG_ON_PAGE(!PageHead(page), page);
> VM_BUG_ON_PAGE(PageCompound(page_tail), page);
> VM_BUG_ON_PAGE(PageLRU(page_tail), page);
> +
> + /* LRU and PageDmaPinned are mutually exclusive: they use the
> + * same fields in struct page, but for different purposes.
> + */
> + VM_BUG_ON_PAGE(PageDmaPinned(page_tail), page);
> VM_BUG_ON(NR_CPUS != 1 &&
> !spin_is_locked(&lruvec_pgdat(lruvec)->lru_lock));
>
> @@ -946,6 +991,12 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
>
> VM_BUG_ON_PAGE(PageLRU(page), page);
>
> + /* LRU and PageDmaPinned are mutually exclusive: they use the
> + * same fields in struct page, but for different purposes.
> + */
> + if (PageDmaPinned(page))
> + return;
> +
> SetPageLRU(page);
> /*
> * Page becomes evictable in two ways:
> --
> 2.19.1


Balbir

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