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	Fri, 10 Jan 2025 04:31:55 +0000
Date: Fri, 10 Jan 2025 04:31:55 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Yang Shi <yang@os.amperecomputing.com>
Cc: Liu Shixin <liushixin2@huawei.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Chengming Zhou <chengming.zhou@linux.dev>,
	Kefeng Wang <wangkefeng.wang@huawei.com>,
	Nanyong Sun <sunnanyong@huawei.com>,
	Muchun Song <muchun.song@linux.dev>,
	Qi Zheng <zhengqi.arch@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: khugepaged: fix call hpage_collapse_scan_file() for
 anonymous vma
Message-ID: <Z4Ciu3E-XjcIga7e@casper.infradead.org>
References: <20250109070059.369257-1-liushixin2@huawei.com>
 <037d4442-4d2d-4aeb-8091-5efffc374d36@os.amperecomputing.com>
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In-Reply-To: <037d4442-4d2d-4aeb-8091-5efffc374d36@os.amperecomputing.com>

On Thu, Jan 09, 2025 at 09:00:24AM -0800, Yang Shi wrote:
> Thanks for catching this. It sounds a little bit weird to have vm_file for
> an anonymous VMA. I'm not sure why we should keep such special case. It
> seems shared mapping is treated as shmem file mapping. So can we set vm_file
> to NULL when mmap'ing /dev/zero for private mapping? Something like:
> 
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 169eed162a7f..fc332efc5c11 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -527,6 +527,7 @@ static int mmap_zero(struct file *file, struct
> vm_area_struct *vma)
>         if (vma->vm_flags & VM_SHARED)
>                 return shmem_zero_setup(vma);
>         vma_set_anonymous(vma);
> +       vma->vm_file = NULL;
>         return 0;
>  }

I'm wary this might cause other bugs somewhere.  rc6 is a bit late to be
introducing such a subtle change.