Re: [PATCH] mm: fix missing wake-up event for FSDAX pages

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Jul 04, 2022 at 11:38:16AM +0100, Matthew Wilcox wrote:
> On Mon, Jul 04, 2022 at 03:40:54PM +0800, Muchun Song wrote:
> > FSDAX page refcounts are 1-based, rather than 0-based: if refcount is
> > 1, then the page is freed.  The FSDAX pages can be pinned through GUP,
> > then they will be unpinned via unpin_user_page() using a folio variant
> > to put the page, however, folio variants did not consider this special
> > case, the result will be to miss a wakeup event (like the user of
> > __fuse_dax_break_layouts()).
> 
> Argh, no.  The 1-based refcounts are a blight on the entire kernel.
> They need to go away, not be pushed into folios as well.  I think

I would be happy if this could go away.

> we're close to having that fixed, but until then, this should do
> the trick?
> 

The following fix looks good to me since it lowers the overhead as
much as possible

Thanks.

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index cc98ab012a9b..4cef5e0f78b6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1129,18 +1129,18 @@ static inline bool is_zone_movable_page(const struct page *page)
>  #if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_FS_DAX)
>  DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
>  
> -bool __put_devmap_managed_page(struct page *page);
> -static inline bool put_devmap_managed_page(struct page *page)
> +bool __put_devmap_managed_page(struct page *page, int refs);
> +static inline bool put_devmap_managed_page(struct page *page, int refs)
>  {
>  	if (!static_branch_unlikely(&devmap_managed_key))
>  		return false;
>  	if (!is_zone_device_page(page))
>  		return false;
> -	return __put_devmap_managed_page(page);
> +	return __put_devmap_managed_page(page, refs);
>  }
>  
>  #else /* CONFIG_ZONE_DEVICE && CONFIG_FS_DAX */
> -static inline bool put_devmap_managed_page(struct page *page)
> +static inline bool put_devmap_managed_page(struct page *page, int refs)
>  {
>  	return false;
>  }
> @@ -1246,7 +1246,7 @@ static inline void put_page(struct page *page)
>  	 * For some devmap managed pages we need to catch refcount transition
>  	 * from 2 to 1:
>  	 */
> -	if (put_devmap_managed_page(&folio->page))
> +	if (put_devmap_managed_page(&folio->page, 1))
>  		return;
>  	folio_put(folio);
>  }
> diff --git a/mm/gup.c b/mm/gup.c
> index d1132b39aa8f..28df02121c78 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -88,7 +88,8 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
>  	 * belongs to this folio.
>  	 */
>  	if (unlikely(page_folio(page) != folio)) {
> -		folio_put_refs(folio, refs);
> +		if (!put_devmap_managed_page(&folio->page, refs))
> +			folio_put_refs(folio, refs);
>  		goto retry;
>  	}
>  
> @@ -177,6 +178,8 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
>  			refs *= GUP_PIN_COUNTING_BIAS;
>  	}
>  
> +	if (put_devmap_managed_page(&folio->page, refs))
> +		return;
>  	folio_put_refs(folio, refs);
>  }
>  
> diff --git a/mm/memremap.c b/mm/memremap.c
> index b870a659eee6..b25e40e3a11e 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -499,7 +499,7 @@ void free_zone_device_page(struct page *page)
>  }
>  
>  #ifdef CONFIG_FS_DAX
> -bool __put_devmap_managed_page(struct page *page)
> +bool __put_devmap_managed_page(struct page *page, int refs)
>  {
>  	if (page->pgmap->type != MEMORY_DEVICE_FS_DAX)
>  		return false;
> @@ -509,7 +509,7 @@ bool __put_devmap_managed_page(struct page *page)
>  	 * refcount is 1, then the page is free and the refcount is
>  	 * stable because nobody holds a reference on the page.
>  	 */
> -	if (page_ref_dec_return(page) == 1)
> +	if (page_ref_sub_return(page, refs) == 1)
>  		wake_up_var(&page->_refcount);
>  	return true;
>  }
> diff --git a/mm/swap.c b/mm/swap.c
> index c6194cfa2af6..94e42a9bab92 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -960,7 +960,7 @@ void release_pages(struct page **pages, int nr)
>  				unlock_page_lruvec_irqrestore(lruvec, flags);
>  				lruvec = NULL;
>  			}
> -			if (put_devmap_managed_page(&folio->page))
> +			if (put_devmap_managed_page(&folio->page, 1))
>  				continue;
>  			if (folio_put_testzero(folio))
>  				free_zone_device_page(&folio->page);
>