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Message-ID: <CACGkMEvz3-G5U3_abVSbMoM=p7iGn54GMXQk-rpvNo4WeLMdFA@mail.gmail.com>
Date: Fri, 16 Aug 2024 10:03:00 +0800
From: Jason Wang <jasowang@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: mst@...hat.com, xuanzhuo@...ux.alibaba.com, eperezma@...hat.com,
maxime.coquelin@...hat.com, xieyongji@...edance.com,
virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
penguin-kernel@...ove.sakura.ne.jp, linux-mm@...ck.org,
akpm@...ux-foundation.org
Subject: Re: [PATCH v2] vduse: avoid using __GFP_NOFAIL
On Fri, Aug 16, 2024 at 8:17 AM Barry Song <21cnbao@...il.com> wrote:
>
> On Thu, Aug 8, 2024 at 5:43 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.
> >
> > 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 as we don't release
> > kernel bounce pages 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")
> > Reviewed-by: Xie Yongji <xieyongji@...edance.com>
> > Tested-by: Xie Yongji <xieyongji@...edance.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
>
> Hi Jason,
>
> Is this the final version to fix __GFP_NOFAIL issue, or will you have a new
> version?
I think so.
> Do you prefer it to go through the mm tree or the drivers subsystem tree?
Either is fine.
> If it's the former, I can send a new version that includes this one in my
> series[1].
>
> [1] https://lore.kernel.org/linux-mm/20240731000155.109583-1-21cnbao@gmail.com/
That's fine.
Thanks
>
> > ---
> > Changes since V1:
> > - Tweak the commit log
> > - Assign map->user_bounce_page to NULL for safety
> > ---
> > drivers/vdpa/vdpa_user/iova_domain.c | 19 +++++++++++--------
> > drivers/vdpa/vdpa_user/iova_domain.h | 1 +
> > 2 files changed, 12 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/vdpa/vdpa_user/iova_domain.c b/drivers/vdpa/vdpa_user/iova_domain.c
> > index 791d38d6284c..58116f89d8da 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 +300,17 @@ 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;
> > }
> > 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
> >
>
> Thanks
> Barry
>
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