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

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Message-ID: <CACycT3sz-OOKKcmH=FgD7gp_Bhi9=nwnkTn0VgRhQBxS2Dp4qw@mail.gmail.com>
Date: Mon, 5 Aug 2024 18:42:07 +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 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.

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.

> > 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>
> > ---
>
> Note for YongJi:
>
> I can only test it without usersapce bounce pages as neither DPDK nor
> libvduse users use that. Would you want to review and have a test for
> this?
>

I can do some tests for it.

Thanks,
Yongji