Re: [PATCH v2 5/6] mm: track gup pages with page->dma_pinned_* fields

Jan Kara
2018-07-02T09:53:31Z

On Sun 01-07-18 17:56:53, 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>

...

> @@ -904,12 +907,24 @@ static inline void get_page(struct page *page)
> */
> VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page);
> page_ref_inc(page);
> +
> + if (unlikely(PageDmaPinned(page)))
> + __get_page_for_pinned_dma(page);
> }
>
> static inline void put_page(struct page *page)
> {
> page = compound_head(page);
>
> + /* Because the page->dma_pinned_* fields are unioned with
> + * page->lru, there is no way to do classical refcount-style
> + * decrement-and-test-for-zero. Instead, PageDmaPinned(page) must
> + * be checked, in order to safely check if we are allowed to decrement
> + * page->dma_pinned_count at all.
> + */
> + if (unlikely(PageDmaPinned(page)))
> + __put_page_for_pinned_dma(page);
> +

These two are just wrong. You cannot make any page reference for
PageDmaPinned() account against a pin count. First, it is just conceptually
wrong as these references need not be long term pins, second, you can
easily race like:

Pinner Random process
get_page(page)
pin_page_for_dma()
put_page(page)
-> oops, page gets unpinned too early

So you really have to create counterpart to get_user_pages() - like
put_user_page() or whatever... It is inconvenient to have to modify all GUP
users but I don't see a way around that.

Honza

> /*
> * For devmap managed pages we need to catch refcount transition from
> * 2 to 1, when refcount reach one it means the page is free and we
> diff --git a/mm/gup.c b/mm/gup.c
> index 73f0b3316fa7..e5c0104fd234 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);
> +
> + if (ret == 0) {
> + /* Avoid problems later, when freeing the page: */
> + ClearPageActive(page);
> + ClearPageUnevictable(page);
> +
> + /* counteract isolate_lru_page's effects: */
> + put_page(page);
> +
> + 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]);
> +
> return i ? i : err;
> }
>
> @@ -1843,7 +1892,7 @@ 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;
> @@ -1864,6 +1913,9 @@ 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]);
> +
> 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 e6f0d5ef320a..510d442647c2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2062,6 +2062,11 @@ 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.
> + */
> + 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));
> @@ -2079,6 +2084,8 @@ 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_tail), page);
> +
> SetPageLRU(page);
> add_page_to_lru_list(page, lruvec, page_lru(page));
> }
> diff --git a/mm/swap.c b/mm/swap.c
> index 26fc9b5f1b6c..09ba61300d06 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -52,6 +52,43 @@ static DEFINE_PER_CPU(struct pagevec, lru_lazyfree_pvecs);
> static DEFINE_PER_CPU(struct pagevec, activate_page_pvecs);
> #endif
>
> +void __get_page_for_pinned_dma(struct page *page)
> +{
> + struct zone *zone = page_zone(page);
> +
> + spin_lock(zone_gup_lock(zone));
> +
> + if (PageDmaPinned(page))
> + atomic_inc(&page->dma_pinned_count);
> +
> + spin_unlock(zone_gup_lock(zone));
> +}
> +EXPORT_SYMBOL(__get_page_for_pinned_dma);
> +
> +void __put_page_for_pinned_dma(struct page *page)
> +{
> + struct zone *zone = page_zone(page);
> +
> + if (atomic_dec_and_test(&page->dma_pinned_count)) {
> + spin_lock(zone_gup_lock(zone));
> +
> + VM_BUG_ON_PAGE(PageLRU(page), page);
> +
> + /* 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));
> + }
> +}
> +EXPORT_SYMBOL(__put_page_for_pinned_dma);
> +
> /*
> * This path almost never happens for VM activity - pages are normally
> * freed via pagevecs. But it gets used by networking.
> @@ -824,6 +861,11 @@ 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));
>
> @@ -863,6 +905,12 @@ 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:
> --
> 2.18.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR