Re: [PATCH] vduse: avoid using __GFP_NOFAIL

[Barry Song <21cnbao@gmail.com>]

On Mon, Aug 5, 2024 at 8:21 PM Jason Wang <jasowang@redhat.com> 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.
> ""“

the current code will result in returning a NULL pointer but
not a busy-loop.

static inline struct page *
__alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
                                                struct alloc_context *ac)
{
        ...
        /*
         * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
         * we always retry
         */
        if (gfp_mask & __GFP_NOFAIL) {
                /*
                 * All existing users of the __GFP_NOFAIL are blockable, so warn
                 * of any new users that actually require GFP_NOWAIT
                 */
                if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
                        goto fail;
                ...
        }
        ...
fail:
        warn_alloc(gfp_mask, ac->nodemask,
                        "page allocation failure: order:%u", order);
got_pg:
        return page;
}


We have two choices to address the issue:
1. busy-loop
2. BUG_ON

the below patch chose 2:
https://lore.kernel.org/linux-mm/20240731000155.109583-5-21cnbao@gmail.com/

>
> 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
> 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
> 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
>