Re: [PATCH] mm/up: combine put_compound_head() and unpin_user_page()

[John Hubbard <jhubbard@nvidia.com>]

On 12/9/20 11:13 AM, Jason Gunthorpe wrote:
> These functions accomplish the same thing but have different
> implementations.
> 
> unpin_user_page() has a bug where it calls mod_node_page_state() after
> calling put_page() which creates a risk that the page could have been
> hot-uplugged from the system.
> 
> Fix this by using put_compound_head() as the only implementation.
> 
> __unpin_devmap_managed_user_page() and related can be deleted as well in
> favour of the simpler, but slower, version in put_compound_head() that has
> an extra atomic page_ref_sub, but always calls put_page() which internally
> contains the special devmap code.
> 
> Move put_compound_head() to be directly after try_grab_compound_head() so
> people can find it in future.
> 
> Fixes: 1970dc6f5226 ("mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting")
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   mm/gup.c | 103 +++++++++++++------------------------------------------
>   1 file changed, 23 insertions(+), 80 deletions(-)
> 

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

With a couple of minor notes below:

> With Matt's folio idea I'd next to go to make a
>    put_folio(folio, refs)
> 
> Which would cleanly eliminate that extra atomic here without duplicating the
> devmap special case.
> 
> This should also be called 'ungrab_compound_head' as we seem to be using the
> word 'grab' to mean 'pin or get' depending on GUP flags.
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 98eb8e6d2609c3..7b33b7d4b324d7 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -123,6 +123,28 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>   	return NULL;
>   }
>   
> +static void put_compound_head(struct page *page, int refs, unsigned int flags)
> +{

It might be nice to rename "page" to "head", here.

While reading this I toyed with the idea of having this at the top:

	VM_BUG_ON_PAGE(compound_head(page) != page, page);

...but it's overkill in a static function with pretty clear call sites. So I
think it's just right as-is.


> +	if (flags & FOLL_PIN) {
> +		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
> +				    refs);
> +
> +		if (hpage_pincount_available(page))
> +			hpage_pincount_sub(page, refs);
> +		else
> +			refs *= GUP_PIN_COUNTING_BIAS;
> +	}
> +
> +	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
> +	/*
> +	 * Calling put_page() for each ref is unnecessarily slow. Only the last
> +	 * ref needs a put_page().
> +	 */
> +	if (refs > 1)
> +		page_ref_sub(page, refs - 1);
> +	put_page(page);
> +}
> +
>   /**
>    * try_grab_page() - elevate a page's refcount by a flag-dependent amount
>    *
> @@ -177,41 +199,6 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
>   	return true;
>   }
>   
> -#ifdef CONFIG_DEV_PAGEMAP_OPS
> -static bool __unpin_devmap_managed_user_page(struct page *page)
> -{
> -	int count, refs = 1;
> -
> -	if (!page_is_devmap_managed(page))
> -		return false;
> -
> -	if (hpage_pincount_available(page))
> -		hpage_pincount_sub(page, 1);
> -	else
> -		refs = GUP_PIN_COUNTING_BIAS;
> -
> -	count = page_ref_sub_return(page, refs);
> -
> -	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
> -	/*
> -	 * devmap page refcounts are 1-based, rather than 0-based: if
> -	 * refcount is 1, then the page is free and the refcount is
> -	 * stable because nobody holds a reference on the page.
> -	 */
> -	if (count == 1)
> -		free_devmap_managed_page(page);
> -	else if (!count)
> -		__put_page(page);
> -
> -	return true;
> -}
> -#else
> -static bool __unpin_devmap_managed_user_page(struct page *page)
> -{
> -	return false;
> -}
> -#endif /* CONFIG_DEV_PAGEMAP_OPS */
> -

Wow, getting rid of that duplication is beautiful!

thanks,
-- 
John Hubbard
NVIDIA

>   /**
>    * unpin_user_page() - release a dma-pinned page
>    * @page:            pointer to page to be released
> @@ -223,28 +210,7 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
>    */
>   void unpin_user_page(struct page *page)
>   {
> -	int refs = 1;
> -
> -	page = compound_head(page);
> -
> -	/*
> -	 * For devmap managed pages we need to catch refcount transition from
> -	 * GUP_PIN_COUNTING_BIAS to 1, when refcount reach one it means the
> -	 * page is free and we need to inform the device driver through
> -	 * callback. See include/linux/memremap.h and HMM for details.
> -	 */
> -	if (__unpin_devmap_managed_user_page(page))
> -		return;
> -
> -	if (hpage_pincount_available(page))
> -		hpage_pincount_sub(page, 1);
> -	else
> -		refs = GUP_PIN_COUNTING_BIAS;
> -
> -	if (page_ref_sub_and_test(page, refs))
> -		__put_page(page);
> -
> -	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
> +	put_compound_head(compound_head(page), 1, FOLL_PIN);
>   }
>   EXPORT_SYMBOL(unpin_user_page);
>   
> @@ -2062,29 +2028,6 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
>    * This code is based heavily on the PowerPC implementation by Nick Piggin.
>    */
>   #ifdef CONFIG_HAVE_FAST_GUP
> -
> -static void put_compound_head(struct page *page, int refs, unsigned int flags)
> -{
> -	if (flags & FOLL_PIN) {
> -		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
> -				    refs);
> -
> -		if (hpage_pincount_available(page))
> -			hpage_pincount_sub(page, refs);
> -		else
> -			refs *= GUP_PIN_COUNTING_BIAS;
> -	}
> -
> -	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
> -	/*
> -	 * Calling put_page() for each ref is unnecessarily slow. Only the last
> -	 * ref needs a put_page().
> -	 */
> -	if (refs > 1)
> -		page_ref_sub(page, refs - 1);
> -	put_page(page);
> -}
> -
>   #ifdef CONFIG_GUP_GET_PTE_LOW_HIGH
>   
>   /*
>