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Date:   Sun, 2 May 2021 09:34:11 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Steven Price <steven.price@arm.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Aili Yao <yaoaili@kingsoft.com>, Jiri Bohac <jbohac@suse.cz>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        linux-hyperv@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v1 7/7] fs/proc/kcore: use page_offline_(freeze|unfreeze)
Message-ID: <YI5H4yV/c6ReuIDt@kernel.org>
References: <20210429122519.15183-1-david@redhat.com>
 <20210429122519.15183-8-david@redhat.com>
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On Thu, Apr 29, 2021 at 02:25:19PM +0200, David Hildenbrand wrote:
> Let's properly synchronize with drivers that set PageOffline(). Unfreeze
> every now and then, so drivers that want to set PageOffline() can make
> progress.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  fs/proc/kcore.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 92ff1e4436cb..3d7531f47389 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -311,6 +311,7 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
>  static ssize_t
>  read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  {
> +	size_t page_offline_frozen = 0;
>  	char *buf = file->private_data;
>  	size_t phdrs_offset, notes_offset, data_offset;
>  	size_t phdrs_len, notes_len;
> @@ -509,6 +510,18 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  			pfn = __pa(start) >> PAGE_SHIFT;
>  			page = pfn_to_online_page(pfn);

Can't this race with page offlining for the first time we get here?
 
> +			/*
> +			 * Don't race against drivers that set PageOffline()
> +			 * and expect no further page access.
> +			 */
> +			if (page_offline_frozen == MAX_ORDER_NR_PAGES) {
> +				page_offline_unfreeze();
> +				page_offline_frozen = 0;
> +				cond_resched();
> +			}
> +			if (!page_offline_frozen++)
> +				page_offline_freeze();
> +

Don't we need to freeze before doing pfn_to_online_page()?

>  			/*
>  			 * Don't read offline sections, logically offline pages
>  			 * (e.g., inflated in a balloon), hwpoisoned pages,
> @@ -565,6 +578,8 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  	}
>  
>  out:
> +	if (page_offline_frozen)
> +		page_offline_unfreeze();
>  	up_read(&kclist_lock);
>  	if (ret)
>  		return ret;
> -- 
> 2.30.2
> 

-- 
Sincerely yours,
Mike.