Re: [mm-unstable PATCH v4 1/9] mm/hugetlb: check gigantic_page_runtime_supported() in return_unused_surplus_pages()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/7/4 9:33, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> I found a weird state of 1GB hugepage pool, caused by the following
> procedure:
> 
>   - run a process reserving all free 1GB hugepages,
>   - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
>     /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
>   - kill the reserving process.
> 
> , then all the hugepages are free *and* surplus at the same time.
> 
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>   3
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
>   3
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
>   0
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
>   3
> 
> This state is resolved by reserving and allocating the pages then
> freeing them again, so this seems not to result in serious problem.
> But it's a little surprising (shrinking pool suddenly fails).
> 
> This behavior is caused by hstate_is_gigantic() check in
> return_unused_surplus_pages(). This was introduced so long ago in 2008
> by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
> at that time the gigantic pages were not supposed to be allocated/freed
> at run-time.  Now kernel can support runtime allocation/free, so let's
> check gigantic_page_runtime_supported() together.
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>

This patch looks good to me with a few question below.

> ---
> v2 -> v3:
> - Fixed typo in patch description,
> - add !gigantic_page_runtime_supported() check instead of removing
>   hstate_is_gigantic() check (suggested by Miaohe and Muchun)
> - add a few more !gigantic_page_runtime_supported() check in
>   set_max_huge_pages() (by Mike).
> ---
>  mm/hugetlb.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 2a554f006255..bdc4499f324b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2432,8 +2432,7 @@ static void return_unused_surplus_pages(struct hstate *h,
>  	/* Uncommit the reservation */
>  	h->resv_huge_pages -= unused_resv_pages;
>  
> -	/* Cannot return gigantic pages currently */
> -	if (hstate_is_gigantic(h))
> +	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
>  		goto out;
>  
>  	/*
> @@ -3315,7 +3314,8 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  	 * the user tries to allocate gigantic pages but let the user free the
>  	 * boottime allocated gigantic pages.
>  	 */
> -	if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
> +	if (hstate_is_gigantic(h) && (!IS_ENABLED(CONFIG_CONTIG_ALLOC) ||
> +				      !gigantic_page_runtime_supported())) {
>  		if (count > persistent_huge_pages(h)) {
>  			spin_unlock_irq(&hugetlb_lock);
>  			mutex_unlock(&h->resize_lock);
> @@ -3363,6 +3363,19 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  			goto out;
>  	}
>  
> +	/*
> +	 * We can not decrease gigantic pool size if runtime modification
> +	 * is not supported.
> +	 */
> +	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported()) {
> +		if (count < persistent_huge_pages(h)) {
> +			spin_unlock_irq(&hugetlb_lock);
> +			mutex_unlock(&h->resize_lock);
> +			NODEMASK_FREE(node_alloc_noretry);
> +			return -EINVAL;
> +		}
> +	}

With above change, we're not allowed to decrease the pool size now. But it was allowed previously
even if !gigantic_page_runtime_supported. Does this will break user?

And it seems it's not allowed to adjust the max_huge_pages now if !gigantic_page_runtime_supported
for gigantic huge page. Should we just return for such case as there should be nothing to do now?
Or am I miss something?

Thanks!

> +
>  	/*
>  	 * Decrease the pool size
>  	 * First return free pages to the buddy allocator (being careful
>