Re: [PATCH v1 7/7] fs/proc/kcore: use page_offline_(freeze|unfreeze)

[Mike Rapoport <rppt@kernel.org>]

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.