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

Mike Rapoport
2021-05-02T06:34:26Z

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.