linux-kernel - Re: [PATCH] vduse: avoid using __GFP

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Message-ID: <CACycT3sHT-izwAKzxAWPbqGFgyf82WxkHHOrp1SjWa+HE01mCg@mail.gmail.com>
Date: Wed, 7 Aug 2024 11:12:55 +0800
From: Yongji Xie <xieyongji@...edance.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Maxime Coquelin <maxime.coquelin@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Eugenio Perez Martin <eperezma@...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 Wed, Aug 7, 2024 at 10:39 AM Jason Wang <jasowang@...hat.com> wrote:
>
> On Tue, Aug 6, 2024 at 11:10 AM Yongji Xie <xieyongji@...edance.com> wrote:
> >
> > On Tue, Aug 6, 2024 at 10:28 AM Jason Wang <jasowang@...hat.com> wrote:
> > >
> > > On Mon, Aug 5, 2024 at 6:42 PM Yongji Xie <xieyongji@...edance.com> wrote:
> > > >
> > > > On Mon, Aug 5, 2024 at 4:24 PM Jason Wang <jasowang@...hat.com> wrote:
> > > > >
> > > > > 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.
> > > > > > """
> > > > > >
> > > >
> > > > I think the problem is it's hard to do the error handling in
> > > > fops->release() currently.
> > >
> > > vduse_dev_dereg_umem() should be the same, it's very hard to allow it to fail.
> > >
> > > >
> > > > So can we temporarily hold the user page refcount, and release it when
> > > > vduse_dev_open()/vduse_domain_release() is executed. The kernel page
> > > > allocation and memcpy can be done in vduse_dev_open() which allows
> > > > some error handling.
> > >
> > > Just to make sure I understand this, the free is probably not the big
> > > issue but the allocation itself.
> > >
> >
> > Yes, so defer the allocation might be a solution.
>
> Would you mind posting a patch for this?
>
> >
> > > And if we do the memcpy() in open(), it seems to be a subtle userspace
> > > noticeable change? (Or I don't get how copying in vduse_dev_open() can
> > > help here).
> > >
> >
> > Maybe we don't need to do the copy in open(). We can hold the user
> > page refcount until the inflight I/O is completed. That means the
> > allocation of new kernel pages can be done in
> > vduse_domain_map_bounce_page() and the release of old user pages can
> > be done in vduse_domain_unmap_bounce_page().
>
> This seems to be a subtle userspace noticeable behaviour?
>

Yes, userspace needs to ensure that it does not reuse the old user
pages for other purposes before vduse_dev_dereg_umem() returns
successfully. The vduse_dev_dereg_umem() will only return successfully
when there is no inflight I/O which means we don't need to allocate
extra kernel pages to store data. If we can't accept this, then your
current patch might be the most suitable. I will test this patch
first.

Thanks,
Yongji