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Message-ID: <CACycT3uM1jSdqFT0LGqy1zXZkWF8BNQN=8EMKYMoyP_wjRtsng@mail.gmail.com>
Date: Wed, 7 Aug 2024 14:52:19 +0800
From: Yongji Xie <xieyongji@...edance.com>
To: Jason Wang <jasowang@...hat.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
Eugenio Perez Martin <eperezma@...hat.com>, Maxime Coquelin <maxime.coquelin@...hat.com>,
virtualization@...ts.linux.dev, linux-kernel <linux-kernel@...r.kernel.org>,
21cnbao@...il.com, penguin-kernel@...ove.sakura.ne.jp, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] vduse: avoid using __GFP_NOFAIL
On Mon, Aug 5, 2024 at 4:21 PM Jason Wang <jasowang@...hat.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.
> ""“
>
> 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@...hat.com>
> ---
Reviewed-by: Xie Yongji <xieyongji@...edance.com>
Tested-by: Xie Yongji <xieyongji@...edance.com>
Have tested it with qemu-storage-daemon [1]:
$ qemu-storage-daemon \
--chardev socket,id=charmonitor,path=/tmp/qmp.sock,server=on,wait=off \
--monitor chardev=charmonitor \
--blockdev driver=host_device,cache.direct=on,aio=native,filename=/dev/nullb0,node-name=disk0
\
--export type=vduse-blk,id=vduse-test,name=vduse-test,node-name=disk0,writable=on
[1] https://github.com/bytedance/qemu/tree/vduse-umem
> 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);
map->user_bounce_page = NULL?
Thanks,
Yongji
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