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

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Message-ID: <CACGkMEvNyB_+fHV5aAeCQhebA8aF8+9hnjkjeEzt5bgrVgqw5Q@mail.gmail.com>
Date: Mon, 5 Aug 2024 16:23:54 +0800
From: Jason Wang <jasowang@...hat.com>
To: xieyongji@...edance.com
Cc: maxime.coquelin@...hat.com, xuanzhuo@...ux.alibaba.com, 
	jasowang@...hat.com, mst@...hat.com, eperezma@...hat.com, 
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	21cnbao@...il.com, penguin-kernel@...ove.sakura.ne.jp, linux-mm@...ck.org, 
	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>
> ---

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?

Thanks