Re: [PATCH v13 2/7] mm: factor helpers for memory_failure_dev_pagemap

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Tue, Apr 19, 2022 at 12:50:40PM +0800, Shiyang Ruan wrote:
> memory_failure_dev_pagemap code is a bit complex before introduce RMAP
> feature for fsdax.  So it is needed to factor some helper functions to
> simplify these code.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>

Thanks for the refactoring.  As I commented to 0/7, the conflict with
"mm/hwpoison: fix race between hugetlb free/demotion and memory_failure_hugetlb()"
can be trivially resolved.

Another few comment below ...

> ---
>  mm/memory-failure.c | 157 ++++++++++++++++++++++++--------------------
>  1 file changed, 87 insertions(+), 70 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index e3fbff5bd467..7c8c047bfdc8 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1498,6 +1498,90 @@ static int try_to_split_thp_page(struct page *page, const char *msg)
>  	return 0;
>  }
> 
> +static void unmap_and_kill(struct list_head *to_kill, unsigned long pfn,
> +		struct address_space *mapping, pgoff_t index, int flags)
> +{
> +	struct to_kill *tk;
> +	unsigned long size = 0;
> +
> +	list_for_each_entry(tk, to_kill, nd)
> +		if (tk->size_shift)
> +			size = max(size, 1UL << tk->size_shift);
> +
> +	if (size) {
> +		/*
> +		 * Unmap the largest mapping to avoid breaking up device-dax
> +		 * mappings which are constant size. The actual size of the
> +		 * mapping being torn down is communicated in siginfo, see
> +		 * kill_proc()
> +		 */
> +		loff_t start = (index << PAGE_SHIFT) & ~(size - 1);
> +
> +		unmap_mapping_range(mapping, start, size, 0);
> +	}
> +
> +	kill_procs(to_kill, flags & MF_MUST_KILL, false, pfn, flags);
> +}
> +
> +static int mf_generic_kill_procs(unsigned long long pfn, int flags,
> +		struct dev_pagemap *pgmap)
> +{
> +	struct page *page = pfn_to_page(pfn);
> +	LIST_HEAD(to_kill);
> +	dax_entry_t cookie;
> +	int rc = 0;
> +
> +	/*
> +	 * Pages instantiated by device-dax (not filesystem-dax)
> +	 * may be compound pages.
> +	 */
> +	page = compound_head(page);
> +
> +	/*
> +	 * Prevent the inode from being freed while we are interrogating
> +	 * the address_space, typically this would be handled by
> +	 * lock_page(), but dax pages do not use the page lock. This
> +	 * also prevents changes to the mapping of this pfn until
> +	 * poison signaling is complete.
> +	 */
> +	cookie = dax_lock_page(page);
> +	if (!cookie)
> +		return -EBUSY;
> +
> +	if (hwpoison_filter(page)) {
> +		rc = -EOPNOTSUPP;
> +		goto unlock;
> +	}
> +
> +	if (pgmap->type == MEMORY_DEVICE_PRIVATE) {
> +		/*
> +		 * TODO: Handle HMM pages which may need coordination
> +		 * with device-side memory.
> +		 */
> +		return -EBUSY;

Don't we need to go to dax_unlock_page() as the origincal code do?

> +	}
> +
> +	/*
> +	 * Use this flag as an indication that the dax page has been
> +	 * remapped UC to prevent speculative consumption of poison.
> +	 */
> +	SetPageHWPoison(page);
> +
> +	/*
> +	 * Unlike System-RAM there is no possibility to swap in a
> +	 * different physical page at a given virtual address, so all
> +	 * userspace consumption of ZONE_DEVICE memory necessitates
> +	 * SIGBUS (i.e. MF_MUST_KILL)
> +	 */
> +	flags |= MF_ACTION_REQUIRED | MF_MUST_KILL;
> +	collect_procs(page, &to_kill, true);
> +
> +	unmap_and_kill(&to_kill, pfn, page->mapping, page->index, flags);
> +unlock:
> +	dax_unlock_page(page, cookie);
> +	return rc;
> +}
> +
>  /*
>   * Called from hugetlb code with hugetlb_lock held.
>   *
> @@ -1644,12 +1728,8 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>  		struct dev_pagemap *pgmap)
>  {
>  	struct page *page = pfn_to_page(pfn);
> -	unsigned long size = 0;
> -	struct to_kill *tk;
>  	LIST_HEAD(tokill);

Is this variable unused in this function?

Thanks,
Naoya Horiguchi