Re: [PATCH v2 2/4] mm/hugetlb: Setup hugetlb_falloc during fallocate hole punch

[Dave Hansen <dave.hansen@intel.com>]

On 10/20/2015 04:52 PM, Mike Kravetz wrote:
>  	if (hole_end > hole_start) {
>  		struct address_space *mapping = inode->i_mapping;
> +		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(hugetlb_falloc_waitq);
> +		/*
> +		 * Page faults on the area to be hole punched must be stopped
> +		 * during the operation.  Initialize struct and have
> +		 * inode->i_private point to it.
> +		 */
> +		struct hugetlb_falloc hugetlb_falloc = {
> +			.waitq = &hugetlb_falloc_waitq,
> +			.start = hole_start >> hpage_shift,
> +			.end = hole_end >> hpage_shift
> +		};
...
> @@ -527,6 +550,12 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>  						hole_end  >> PAGE_SHIFT);
>  		i_mmap_unlock_write(mapping);
>  		remove_inode_hugepages(inode, hole_start, hole_end);
> +
> +		spin_lock(&inode->i_lock);
> +		inode->i_private = NULL;
> +		wake_up_all(&hugetlb_falloc_waitq);
> +		spin_unlock(&inode->i_lock);

I see the shmem code doing something similar.  But, in the end, we're
passing the stack-allocated 'hugetlb_falloc_waitq' over to the page
faulting thread.  Is there something subtle that keeps
'hugetlb_falloc_waitq' from becoming invalid while the other task is
sleeping?

That wake_up_all() obviously can't sleep, but it seems like the faulting
thread's finish_wait() *HAS* to run before wake_up_all() can return.

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