Re: [PATCH v2 1/4] hugetlb: add demote hugetlb page sysfs interfaces

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 23 Sep 2021 10:53:44 -0700 Mike Kravetz <mike.kravetz@oracle.com> wrote:

> Two new sysfs files are added to demote hugtlb pages.  These files are
> both per-hugetlb page size and per node.  Files are:
>   demote_size - The size in Kb that pages are demoted to. (read-write)
>   demote - The number of huge pages to demote. (write-only)
> 
> By default, demote_size is the next smallest huge page size.  Valid huge
> page sizes less than huge page size may be written to this file.  When
> huge pages are demoted, they are demoted to this size.
> 
> Writing a value to demote will result in an attempt to demote that
> number of hugetlb pages to an appropriate number of demote_size pages.
> 
> NOTE: Demote interfaces are only provided for huge page sizes if there
> is a smaller target demote huge page size.  For example, on x86 1GB huge
> pages will have demote interfaces.  2MB huge pages will not have demote
> interfaces.
> 
> This patch does not provide full demote functionality.  It only provides
> the sysfs interfaces.
> 
> It also provides documentation for the new interfaces.
> 
> ...
>
> +static ssize_t demote_store(struct kobject *kobj,
> +	       struct kobj_attribute *attr, const char *buf, size_t len)
> +{
> +	unsigned long nr_demote;
> +	unsigned long nr_available;
> +	nodemask_t nodes_allowed, *n_mask;
> +	struct hstate *h;
> +	int err;
> +	int nid;
> +
> +	err = kstrtoul(buf, 10, &nr_demote);
> +	if (err)
> +		return err;
> +	h = kobj_to_hstate(kobj, &nid);
> +
> +	/* Synchronize with other sysfs operations modifying huge pages */
> +	mutex_lock(&h->resize_lock);
> +
> +	spin_lock_irq(&hugetlb_lock);
> +	if (nid != NUMA_NO_NODE) {
> +		nr_available = h->free_huge_pages_node[nid];
> +		init_nodemask_of_node(&nodes_allowed, nid);
> +		n_mask = &nodes_allowed;
> +	} else {
> +		nr_available = h->free_huge_pages;
> +		n_mask = &node_states[N_MEMORY];
> +	}
> +	nr_available -= h->resv_huge_pages;
> +	if (nr_available <= 0)
> +		goto out;
> +	nr_demote = min(nr_available, nr_demote);
> +
> +	while (nr_demote) {
> +		if (!demote_pool_huge_page(h, n_mask))
> +			break;
> +
> +		/*
> +		 * We may have dropped the lock in the routines to
> +		 * demote/free a page.  Recompute nr_demote as counts could
> +		 * have changed and we want to make sure we do not demote
> +		 * a reserved huge page.
> +		 */

This comment doesn't become true until patch #4, and is a bit confusing
in patch #1.  Also, saying "the lock" is far less helpful than saying
"hugetlb_lock"!


> +		nr_demote--;
> +		if (nid != NUMA_NO_NODE)
> +			nr_available = h->free_huge_pages_node[nid];
> +		else
> +			nr_available = h->free_huge_pages;
> +		nr_available -= h->resv_huge_pages;
> +		if (nr_available <= 0)
> +			nr_demote = 0;
> +		else
> +			nr_demote = min(nr_available, nr_demote);
> +	}
> +
> +out:
> +	spin_unlock_irq(&hugetlb_lock);

How long can we spend with IRQs disabled here (after patch #4!)?

> +	mutex_unlock(&h->resize_lock);
> +
> +	return len;
> +}
> +HSTATE_ATTR_WO(demote);
> +