Re: [PATCH] vduse: avoid using __GFP_NOFAIL

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 05, 2024 at 04:21:06PM +0800, Jason Wang wrote:
> Barry said [1]:
> 
> """
> mm doesn't support non-blockable __GFP_NOFAIL allocation. Because
> __GFP_NOFAIL without direct reclamation may just result in a busy
> loop within non-sleepable contexts.
> ""“
> 
> Unfortuantely, we do that under read lock. A possible way to fix that
> is to move the pages allocation out of the lock into the caller, but
> having to allocate a huge number of pages and auxiliary page array
> seems to be problematic as well per Tetsuon [2]:
> 
> """
> You should implement proper error handling instead of using
> __GFP_NOFAIL if count can become large.
> """
> 
> So I choose another way, which does not release kernel bounce pages
> when user tries to register usersapce bounce pages. Then we don't need

userspace

> to do allocation in the path which is not expected to be fail (e.g in
> the release). We pay this for more memory usage but further

what does "we pay this for more memory usage" mean?
Do you mean "we pay for this by using more memory"?
How much more?

> optimizations could be done on top.
> 
> [1] https://lore.kernel.org/all/CACGkMEtcOJAA96SF9B8m-nZ1X04-XZr+nq8ZQ2saLnUdfOGOLg@mail.gmail.com/T/#m3caef86a66ea6318ef94f9976ddb3a0ccfe6fcf8
> [2] https://lore.kernel.org/all/CACGkMEtcOJAA96SF9B8m-nZ1X04-XZr+nq8ZQ2saLnUdfOGOLg@mail.gmail.com/T/#m7ad10eaba48ade5abf2d572f24e185d9fb146480
> 
> Fixes: 6c77ed22880d ("vduse: Support using userspace pages as bounce buffer")
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/vdpa/vdpa_user/iova_domain.c | 18 ++++++++++--------
>  drivers/vdpa/vdpa_user/iova_domain.h |  1 +
>  2 files changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vdpa/vdpa_user/iova_domain.c b/drivers/vdpa/vdpa_user/iova_domain.c
> index 791d38d6284c..933d2f7cd49a 100644
> --- a/drivers/vdpa/vdpa_user/iova_domain.c
> +++ b/drivers/vdpa/vdpa_user/iova_domain.c
> @@ -162,6 +162,7 @@ static void vduse_domain_bounce(struct vduse_iova_domain *domain,
>  				enum dma_data_direction dir)
>  {
>  	struct vduse_bounce_map *map;
> +	struct page *page;
>  	unsigned int offset;
>  	void *addr;
>  	size_t sz;
> @@ -178,7 +179,10 @@ static void vduse_domain_bounce(struct vduse_iova_domain *domain,
>  			    map->orig_phys == INVALID_PHYS_ADDR))
>  			return;
>  
> -		addr = kmap_local_page(map->bounce_page);
> +		page = domain->user_bounce_pages ?
> +		       map->user_bounce_page : map->bounce_page;
> +
> +		addr = kmap_local_page(page);
>  		do_bounce(map->orig_phys + offset, addr + offset, sz, dir);
>  		kunmap_local(addr);
>  		size -= sz;
> @@ -270,9 +274,8 @@ int vduse_domain_add_user_bounce_pages(struct vduse_iova_domain *domain,
>  				memcpy_to_page(pages[i], 0,
>  					       page_address(map->bounce_page),
>  					       PAGE_SIZE);
> -			__free_page(map->bounce_page);
>  		}
> -		map->bounce_page = pages[i];
> +		map->user_bounce_page = pages[i];
>  		get_page(pages[i]);
>  	}
>  	domain->user_bounce_pages = true;
> @@ -297,17 +300,16 @@ void vduse_domain_remove_user_bounce_pages(struct vduse_iova_domain *domain)
>  		struct page *page = NULL;
>  
>  		map = &domain->bounce_maps[i];
> -		if (WARN_ON(!map->bounce_page))
> +		if (WARN_ON(!map->user_bounce_page))
>  			continue;
>  
>  		/* Copy user page to kernel page if it's in use */
>  		if (map->orig_phys != INVALID_PHYS_ADDR) {
> -			page = alloc_page(GFP_ATOMIC | __GFP_NOFAIL);
> +			page = map->bounce_page;
>  			memcpy_from_page(page_address(page),
> -					 map->bounce_page, 0, PAGE_SIZE);
> +					 map->user_bounce_page, 0, PAGE_SIZE);
>  		}
> -		put_page(map->bounce_page);
> -		map->bounce_page = page;
> +		put_page(map->user_bounce_page);
>  	}
>  	domain->user_bounce_pages = false;
>  out:
> diff --git a/drivers/vdpa/vdpa_user/iova_domain.h b/drivers/vdpa/vdpa_user/iova_domain.h
> index f92f22a7267d..7f3f0928ec78 100644
> --- a/drivers/vdpa/vdpa_user/iova_domain.h
> +++ b/drivers/vdpa/vdpa_user/iova_domain.h
> @@ -21,6 +21,7 @@
>  
>  struct vduse_bounce_map {
>  	struct page *bounce_page;
> +	struct page *user_bounce_page;
>  	u64 orig_phys;
>  };
>  
> -- 
> 2.31.1