Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count

[Balbir Singh]

On Thu, Oct 11, 2018 at 11:00:12PM -0700, john.hubbard@xxxxxxxxx wrote:
> From: John Hubbard <jhubbard@xxxxxxxxxx>
> 
> Add two struct page fields that, combined, are unioned with
> struct page->lru. There is no change in the size of
> struct page. These new fields are for type safety and clarity.
> 
> Also add page flag accessors to test, set and clear the new
> page->dma_pinned_flags field.
> 
> The page->dma_pinned_count field will be used in upcoming
> patches
> 
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  include/linux/mm_types.h   | 22 +++++++++++++-----
>  include/linux/page-flags.h | 47 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 63 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5ed8f6292a53..017ab82e36ca 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -78,12 +78,22 @@ struct page {
>  	 */
>  	union {
>  		struct {	/* Page cache and anonymous pages */
> -			/**
> -			 * @lru: Pageout list, eg. active_list protected by
> -			 * zone_lru_lock.  Sometimes used as a generic list
> -			 * by the page owner.
> -			 */
> -			struct list_head lru;
> +			union {
> +				/**
> +				 * @lru: Pageout list, eg. active_list protected
> +				 * by zone_lru_lock.  Sometimes used as a
> +				 * generic list by the page owner.
> +				 */
> +				struct list_head lru;
> +				/* Used by get_user_pages*(). Pages may not be
> +				 * on an LRU while these dma_pinned_* fields
> +				 * are in use.
> +				 */
> +				struct {
> +					unsigned long dma_pinned_flags;
> +					atomic_t      dma_pinned_count;
> +				};
> +			};
>  			/* See page-flags.h for PAGE_MAPPING_FLAGS */
>  			struct address_space *mapping;
>  			pgoff_t index;		/* Our offset within mapping. */
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 74bee8cecf4c..81ed52c3caae 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -425,6 +425,53 @@ static __always_inline int __PageMovable(struct page *page)
>  				PAGE_MAPPING_MOVABLE;
>  }
>  
> +/*
> + * Because page->dma_pinned_flags is unioned with page->lru, any page that
> + * uses these flags must NOT be on an LRU. That's partly enforced by
> + * ClearPageDmaPinned, which gives the page back to LRU.
> + *
> + * PageDmaPinned also corresponds to PageTail (the 0th bit in the first union
> + * of struct page), and this flag is checked without knowing whether it is a
> + * tail page or a PageDmaPinned page. Therefore, start the flags at bit 1 (0x2),
> + * rather than bit 0.
> + */
> +#define PAGE_DMA_PINNED		0x2
> +#define PAGE_DMA_PINNED_FLAGS	(PAGE_DMA_PINNED)
> +

This is really subtle, additional changes to compound_head will need to coordinate
with these flags? Also doesn't this bit need to be unique across all structs in
the union? I guess that is guaranteed by the fact that page == compound_head(page)
as per your assertion, but I've forgotten why that is true. Could you please
add some commentary on that

> +/*
> + * Because these flags are read outside of a lock, ensure visibility between
> + * different threads, by using READ|WRITE_ONCE.
> + */
> +static __always_inline int PageDmaPinnedFlags(struct page *page)
> +{
> +	VM_BUG_ON(page != compound_head(page));
> +	return (READ_ONCE(page->dma_pinned_flags) & PAGE_DMA_PINNED_FLAGS) != 0;
> +}
> +
> +static __always_inline int PageDmaPinned(struct page *page)
> +{
> +	VM_BUG_ON(page != compound_head(page));
> +	return (READ_ONCE(page->dma_pinned_flags) & PAGE_DMA_PINNED) != 0;
> +}
> +
> +static __always_inline void SetPageDmaPinned(struct page *page)
> +{
> +	VM_BUG_ON(page != compound_head(page));

VM_BUG_ON(!list_empty(&page->lru))

> +	WRITE_ONCE(page->dma_pinned_flags, PAGE_DMA_PINNED);
> +}
> +
> +static __always_inline void ClearPageDmaPinned(struct page *page)
> +{
> +	VM_BUG_ON(page != compound_head(page));
> +	VM_BUG_ON_PAGE(!PageDmaPinnedFlags(page), page);
> +
> +	/* This does a WRITE_ONCE to the lru.next, which is also the
> +	 * page->dma_pinned_flags field. So in addition to restoring page->lru,
> +	 * this provides visibility to other threads.
> +	 */
> +	INIT_LIST_HEAD(&page->lru);

This assumes certain things about list_head, why not use the correct
initialization bits.

> +}
> +
>  #ifdef CONFIG_KSM
>  /*
>   * A KSM page is one of those write-protected "shared pages" or "merged pages"
> -- 
> 2.19.1
>