Re: [PATCH v4 4/6] mm: swap: Allow storage of all mTHP orders

[Ryan Roberts]

On 12/03/2024 07:51, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
> 
>> Multi-size THP enables performance improvements by allocating large,
>> pte-mapped folios for anonymous memory. However I've observed that on an
>> arm64 system running a parallel workload (e.g. kernel compilation)
>> across many cores, under high memory pressure, the speed regresses. This
>> is due to bottlenecking on the increased number of TLBIs added due to
>> all the extra folio splitting when the large folios are swapped out.
>>
>> Therefore, solve this regression by adding support for swapping out mTHP
>> without needing to split the folio, just like is already done for
>> PMD-sized THP. This change only applies when CONFIG_THP_SWAP is enabled,
>> and when the swap backing store is a non-rotating block device. These
>> are the same constraints as for the existing PMD-sized THP swap-out
>> support.
>>
>> Note that no attempt is made to swap-in (m)THP here - this is still done
>> page-by-page, like for PMD-sized THP. But swapping-out mTHP is a
>> prerequisite for swapping-in mTHP.
>>
>> The main change here is to improve the swap entry allocator so that it
>> can allocate any power-of-2 number of contiguous entries between [1, (1
>> << PMD_ORDER)]. This is done by allocating a cluster for each distinct
>> order and allocating sequentially from it until the cluster is full.
>> This ensures that we don't need to search the map and we get no
>> fragmentation due to alignment padding for different orders in the
>> cluster. If there is no current cluster for a given order, we attempt to
>> allocate a free cluster from the list. If there are no free clusters, we
>> fail the allocation and the caller can fall back to splitting the folio
>> and allocates individual entries (as per existing PMD-sized THP
>> fallback).
>>
>> The per-order current clusters are maintained per-cpu using the existing
>> infrastructure. This is done to avoid interleving pages from different
>> tasks, which would prevent IO being batched. This is already done for
>> the order-0 allocations so we follow the same pattern.
>>
>> As is done for order-0 per-cpu clusters, the scanner now can steal
>> order-0 entries from any per-cpu-per-order reserved cluster. This
>> ensures that when the swap file is getting full, space doesn't get tied
>> up in the per-cpu reserves.
>>
>> This change only modifies swap to be able to accept any order mTHP. It
>> doesn't change the callers to elide doing the actual split. That will be
>> done in separate changes.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>>  include/linux/swap.h |   8 ++-
>>  mm/swapfile.c        | 167 +++++++++++++++++++++++++------------------
>>  2 files changed, 103 insertions(+), 72 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 0cb082bee717..39b5c18ccc6a 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -268,13 +268,19 @@ struct swap_cluster_info {
>>   */
>>  #define SWAP_NEXT_INVALID	0
>>  
>> +#ifdef CONFIG_THP_SWAP
>> +#define SWAP_NR_ORDERS		(PMD_ORDER + 1)
>> +#else
>> +#define SWAP_NR_ORDERS		1
>> +#endif
>> +
>>  /*
>>   * We assign a cluster to each CPU, so each CPU can allocate swap entry from
>>   * its own cluster and swapout sequentially. The purpose is to optimize swapout
>>   * throughput.
>>   */
>>  struct percpu_cluster {
>> -	unsigned int next; /* Likely next allocation offset */
>> +	unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
>>  };
>>  
>>  struct swap_cluster_list {
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 3828d81aa6b8..61118a090796 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -551,10 +551,12 @@ static void free_cluster(struct swap_info_struct *si, unsigned long idx)
>>  
>>  /*
>>   * The cluster corresponding to page_nr will be used. The cluster will be
>> - * removed from free cluster list and its usage counter will be increased.
>> + * removed from free cluster list and its usage counter will be increased by
>> + * count.
>>   */
>> -static void inc_cluster_info_page(struct swap_info_struct *p,
>> -	struct swap_cluster_info *cluster_info, unsigned long page_nr)
>> +static void add_cluster_info_page(struct swap_info_struct *p,
>> +	struct swap_cluster_info *cluster_info, unsigned long page_nr,
>> +	unsigned long count)
>>  {
>>  	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
>>  
>> @@ -563,9 +565,19 @@ static void inc_cluster_info_page(struct swap_info_struct *p,
>>  	if (cluster_is_free(&cluster_info[idx]))
>>  		alloc_cluster(p, idx);
>>  
>> -	VM_BUG_ON(cluster_count(&cluster_info[idx]) >= SWAPFILE_CLUSTER);
>> +	VM_BUG_ON(cluster_count(&cluster_info[idx]) + count > SWAPFILE_CLUSTER);
>>  	cluster_set_count(&cluster_info[idx],
>> -		cluster_count(&cluster_info[idx]) + 1);
>> +		cluster_count(&cluster_info[idx]) + count);
>> +}
>> +
>> +/*
>> + * The cluster corresponding to page_nr will be used. The cluster will be
>> + * removed from free cluster list and its usage counter will be increased by 1.
>> + */
>> +static void inc_cluster_info_page(struct swap_info_struct *p,
>> +	struct swap_cluster_info *cluster_info, unsigned long page_nr)
>> +{
>> +	add_cluster_info_page(p, cluster_info, page_nr, 1);
>>  }
>>  
>>  /*
>> @@ -595,7 +607,7 @@ static void dec_cluster_info_page(struct swap_info_struct *p,
>>   */
>>  static bool
>>  scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
>> -	unsigned long offset)
>> +	unsigned long offset, int order)
>>  {
>>  	struct percpu_cluster *percpu_cluster;
>>  	bool conflict;
>> @@ -609,24 +621,39 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
>>  		return false;
>>  
>>  	percpu_cluster = this_cpu_ptr(si->percpu_cluster);
>> -	percpu_cluster->next = SWAP_NEXT_INVALID;
>> +	percpu_cluster->next[order] = SWAP_NEXT_INVALID;
>> +	return true;
>> +}
>> +
>> +static inline bool swap_range_empty(char *swap_map, unsigned int start,
>> +				    unsigned int nr_pages)
>> +{
>> +	unsigned int i;
>> +
>> +	for (i = 0; i < nr_pages; i++) {
>> +		if (swap_map[start + i])
>> +			return false;
>> +	}
>> +
>>  	return true;
>>  }
>>  
>>  /*
>> - * Try to get a swap entry from current cpu's swap entry pool (a cluster). This
>> - * might involve allocating a new cluster for current CPU too.
>> + * Try to get a swap entry (or size indicated by order) from current cpu's swap
> 
> IMO, it's not necessary to make mTHP a special case other than base
> page.  So, this can be changed to
> 
>  * Try to get swap entries with specified order from current cpu's swap

Sure, will fix in next version.

> 
>> + * entry pool (a cluster). This might involve allocating a new cluster for
>> + * current CPU too.
>>   */
>>  static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
>> -	unsigned long *offset, unsigned long *scan_base)
>> +	unsigned long *offset, unsigned long *scan_base, int order)
>>  {
>> +	unsigned int nr_pages = 1 << order;
>>  	struct percpu_cluster *cluster;
>>  	struct swap_cluster_info *ci;
>>  	unsigned int tmp, max;
>>  
>>  new_cluster:
>>  	cluster = this_cpu_ptr(si->percpu_cluster);
>> -	tmp = cluster->next;
>> +	tmp = cluster->next[order];
>>  	if (tmp == SWAP_NEXT_INVALID) {
>>  		if (!cluster_list_empty(&si->free_clusters)) {
>>  			tmp = cluster_next(&si->free_clusters.head) *
>> @@ -647,26 +674,27 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
>>  
>>  	/*
>>  	 * Other CPUs can use our cluster if they can't find a free cluster,
>> -	 * check if there is still free entry in the cluster
>> +	 * check if there is still free entry in the cluster, maintaining
>> +	 * natural alignment.
>>  	 */
>>  	max = min_t(unsigned long, si->max, ALIGN(tmp + 1, SWAPFILE_CLUSTER));
>>  	if (tmp < max) {
>>  		ci = lock_cluster(si, tmp);
>>  		while (tmp < max) {
>> -			if (!si->swap_map[tmp])
>> +			if (swap_range_empty(si->swap_map, tmp, nr_pages))
>>  				break;
>> -			tmp++;
>> +			tmp += nr_pages;
>>  		}
>>  		unlock_cluster(ci);
>>  	}
>>  	if (tmp >= max) {
>> -		cluster->next = SWAP_NEXT_INVALID;
>> +		cluster->next[order] = SWAP_NEXT_INVALID;
>>  		goto new_cluster;
>>  	}
>>  	*offset = tmp;
>>  	*scan_base = tmp;
>> -	tmp += 1;
>> -	cluster->next = tmp < max ? tmp : SWAP_NEXT_INVALID;
>> +	tmp += nr_pages;
>> +	cluster->next[order] = tmp < max ? tmp : SWAP_NEXT_INVALID;
>>  	return true;
>>  }
>>  
>> @@ -796,13 +824,14 @@ static bool swap_offset_available_and_locked(struct swap_info_struct *si,
>>  
>>  static int scan_swap_map_slots(struct swap_info_struct *si,
>>  			       unsigned char usage, int nr,
>> -			       swp_entry_t slots[])
>> +			       swp_entry_t slots[], unsigned int nr_pages)
> 
> IMHO, it's better to use order as parameter directly.  We can change the
> parameter of get_swap_pages() too.

I agree that this will make the interface clearer/self documenting. I'll do it
in the next version.

> 
>>  {
>>  	struct swap_cluster_info *ci;
>>  	unsigned long offset;
>>  	unsigned long scan_base;
>>  	unsigned long last_in_cluster = 0;
>>  	int latency_ration = LATENCY_LIMIT;
>> +	int order = ilog2(nr_pages);
>>  	int n_ret = 0;
>>  	bool scanned_many = false;
>>  
>> @@ -817,6 +846,26 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  	 * And we let swap pages go all over an SSD partition.  Hugh
>>  	 */
>>  
>> +	if (nr_pages > 1) {
>> +		/*
>> +		 * Should not even be attempting large allocations when huge
>> +		 * page swap is disabled.  Warn and fail the allocation.
>> +		 */
>> +		if (!IS_ENABLED(CONFIG_THP_SWAP) ||
>> +		    nr_pages > SWAPFILE_CLUSTER ||
>> +		    !is_power_of_2(nr_pages)) {
>> +			VM_WARN_ON_ONCE(1);
>> +			return 0;
>> +		}
>> +
>> +		/*
>> +		 * Swapfile is not block device or not using clusters so unable
>> +		 * to allocate large entries.
>> +		 */
>> +		if (!(si->flags & SWP_BLKDEV) || !si->cluster_info)
>> +			return 0;
>> +	}
>> +
>>  	si->flags += SWP_SCANNING;
>>  	/*
>>  	 * Use percpu scan base for SSD to reduce lock contention on
>> @@ -831,8 +880,11 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  
>>  	/* SSD algorithm */
>>  	if (si->cluster_info) {
>> -		if (!scan_swap_map_try_ssd_cluster(si, &offset, &scan_base))
>> +		if (!scan_swap_map_try_ssd_cluster(si, &offset, &scan_base, order)) {
>> +			if (order > 0)
>> +				goto no_page;
>>  			goto scan;
>> +		}
>>  	} else if (unlikely(!si->cluster_nr--)) {
>>  		if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER) {
>>  			si->cluster_nr = SWAPFILE_CLUSTER - 1;
>> @@ -874,26 +926,30 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  
>>  checks:
>>  	if (si->cluster_info) {
>> -		while (scan_swap_map_ssd_cluster_conflict(si, offset)) {
>> +		while (scan_swap_map_ssd_cluster_conflict(si, offset, order)) {
>>  		/* take a break if we already got some slots */
>>  			if (n_ret)
>>  				goto done;
>>  			if (!scan_swap_map_try_ssd_cluster(si, &offset,
>> -							&scan_base))
>> +							&scan_base, order)) {
>> +				if (order > 0)
>> +					goto no_page;
>>  				goto scan;
>> +			}
>>  		}
>>  	}
>>  	if (!(si->flags & SWP_WRITEOK))
>>  		goto no_page;
>>  	if (!si->highest_bit)
>>  		goto no_page;
>> -	if (offset > si->highest_bit)
>> +	if (order == 0 && offset > si->highest_bit)
> 
> I don't think that we need to check "order == 0" here.  The original
> condition will always be false for "order != 0".

I spent ages looking at this and couldn't quite convince myself that this is
definitely safe. Certainly it would be catastrophic if we modified the returned
offset for a non-order-0 case (the code below assumes order-0 when checking). So
I decided in the end to be safe and add this condition. Looking again, I agree
with you. Will fix in next version.

> 
>>  		scan_base = offset = si->lowest_bit;
>>  
>>  	ci = lock_cluster(si, offset);
>>  	/* reuse swap entry of cache-only swap if not busy. */
>>  	if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
>>  		int swap_was_freed;
>> +		VM_WARN_ON(order > 0);
> 
> Instead of add WARN here, I think that it's better to add WARN at the
> beginning of "scan" label.  We should never scan if "order > 0", it can
> capture even more abnormal status.

OK, will do.

> 
>>  		unlock_cluster(ci);
>>  		spin_unlock(&si->lock);
>>  		swap_was_freed = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY);
>> @@ -905,17 +961,18 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  	}
>>  
>>  	if (si->swap_map[offset]) {
>> +		VM_WARN_ON(order > 0);

And remove this one too? (relying on the one in scan instead)

>>  		unlock_cluster(ci);
>>  		if (!n_ret)
>>  			goto scan;
>>  		else
>>  			goto done;
>>  	}
>> -	WRITE_ONCE(si->swap_map[offset], usage);
>> -	inc_cluster_info_page(si, si->cluster_info, offset);
>> +	memset(si->swap_map + offset, usage, nr_pages);
> 
> Add barrier() here corresponds to original WRITE_ONCE()?
> unlock_cluster(ci) may be NOP for some swap devices.

Yep, good spot!

> 
>> +	add_cluster_info_page(si, si->cluster_info, offset, nr_pages);
>>  	unlock_cluster(ci);
>>  
>> -	swap_range_alloc(si, offset, 1);
>> +	swap_range_alloc(si, offset, nr_pages);
>>  	slots[n_ret++] = swp_entry(si->type, offset);
>>  
>>  	/* got enough slots or reach max slots? */
> 
> If "order > 0", "nr" must be 1.  So, we will "goto done" in the
> following code.

I've deliberately implemented scan_swap_map_slots() so that it allows nr > 1 for
order > 0. And leave it to the higher layers to decide on policy.

> 
>         /* got enough slots or reach max slots? */
>         if ((n_ret == nr) || (offset >= si->highest_bit))
>            goto done;
> 
> We can add VM_WARN_ON() here to capture some abnormal status.

That was actually how I implemented initially. But decided that it doesn't cost
anything to allow nr > 1 for order > 0, and IMHO makes the function easier to
understand because we remove this uneccessary constraint.

> 
>> @@ -936,8 +993,10 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  
>>  	/* try to get more slots in cluster */
>>  	if (si->cluster_info) {
>> -		if (scan_swap_map_try_ssd_cluster(si, &offset, &scan_base))
>> +		if (scan_swap_map_try_ssd_cluster(si, &offset, &scan_base, order))
>>  			goto checks;
>> +		if (order > 0)
>> +			goto done;
> 
> Don't need to add this, if "order > 0", we will never go here.

As per above.

> 
>>  	} else if (si->cluster_nr && !si->swap_map[++offset]) {
>>  		/* non-ssd case, still more slots in cluster? */
>>  		--si->cluster_nr;
>> @@ -964,7 +1023,8 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  	}
>>  
>>  done:
>> -	set_cluster_next(si, offset + 1);
>> +	if (order == 0)
>> +		set_cluster_next(si, offset + 1);
>>  	si->flags -= SWP_SCANNING;
>>  	return n_ret;
>>  
>> @@ -997,38 +1057,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  	return n_ret;
>>  }
>>  
>> -static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
>> -{
>> -	unsigned long idx;
>> -	struct swap_cluster_info *ci;
>> -	unsigned long offset;
>> -
>> -	/*
>> -	 * Should not even be attempting cluster allocations when huge
>> -	 * page swap is disabled.  Warn and fail the allocation.
>> -	 */
>> -	if (!IS_ENABLED(CONFIG_THP_SWAP)) {
>> -		VM_WARN_ON_ONCE(1);
>> -		return 0;
>> -	}
>> -
>> -	if (cluster_list_empty(&si->free_clusters))
>> -		return 0;
>> -
>> -	idx = cluster_list_first(&si->free_clusters);
>> -	offset = idx * SWAPFILE_CLUSTER;
>> -	ci = lock_cluster(si, offset);
>> -	alloc_cluster(si, idx);
>> -	cluster_set_count(ci, SWAPFILE_CLUSTER);
>> -
>> -	memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER);
>> -	unlock_cluster(ci);
>> -	swap_range_alloc(si, offset, SWAPFILE_CLUSTER);
>> -	*slot = swp_entry(si->type, offset);
>> -
>> -	return 1;
>> -}
>> -
>>  static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
>>  {
>>  	unsigned long offset = idx * SWAPFILE_CLUSTER;
>> @@ -1050,8 +1078,8 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
>>  	int n_ret = 0;
>>  	int node;
>>  
>> -	/* Only single cluster request supported */
>> -	WARN_ON_ONCE(n_goal > 1 && size == SWAPFILE_CLUSTER);
>> +	/* Only single THP request supported */
>> +	WARN_ON_ONCE(n_goal > 1 && size > 1);
>>  
>>  	spin_lock(&swap_avail_lock);
>>  
>> @@ -1088,14 +1116,10 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
>>  			spin_unlock(&si->lock);
>>  			goto nextsi;
>>  		}
>> -		if (size == SWAPFILE_CLUSTER) {
>> -			if (si->flags & SWP_BLKDEV)
>> -				n_ret = swap_alloc_cluster(si, swp_entries);
>> -		} else
>> -			n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
>> -						    n_goal, swp_entries);
>> +		n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
>> +					    n_goal, swp_entries, size);
>>  		spin_unlock(&si->lock);
>> -		if (n_ret || size == SWAPFILE_CLUSTER)
>> +		if (n_ret || size > 1)
>>  			goto check_out;
>>  		cond_resched();
>>  
>> @@ -1647,7 +1671,7 @@ swp_entry_t get_swap_page_of_type(int type)
>>  
>>  	/* This is called for allocating swap entry, not cache */
>>  	spin_lock(&si->lock);
>> -	if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry))
>> +	if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry, 1))
>>  		atomic_long_dec(&nr_swap_pages);
>>  	spin_unlock(&si->lock);
>>  fail:
>> @@ -3101,7 +3125,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>>  		p->flags |= SWP_SYNCHRONOUS_IO;
>>  
>>  	if (p->bdev && bdev_nonrot(p->bdev)) {
>> -		int cpu;
>> +		int cpu, i;
>>  		unsigned long ci, nr_cluster;
>>  
>>  		p->flags |= SWP_SOLIDSTATE;
>> @@ -3139,7 +3163,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>>  			struct percpu_cluster *cluster;
>>  
>>  			cluster = per_cpu_ptr(p->percpu_cluster, cpu);
>> -			cluster->next = SWAP_NEXT_INVALID;
>> +			for (i = 0; i < SWAP_NR_ORDERS; i++)
>> +				cluster->next[i] = SWAP_NEXT_INVALID;
>>  		}
>>  	} else {
>>  		atomic_inc(&nr_rotate_swap);
> 
> You also need to check whether we should add swap_entry_size() for some
> functions to optimize for small system.  We may need to add swap_order()
> too.

I was planning to convert swap_entry_size() to swap_entry_order() as part of
switching to pass order instead of nr_pages. There is one other site that uses
swap_entry_size() and needs a size, so was going to just change it to 1 <<
swap_entry_order(). Does that work for you?

I'll do an audit for places to use swap_entry_order() but quick scan just now
suggests that the constant should propagate to all the static functions from
get_swap_pages().

Thanks,
Ryan

> 
> --
> Best Regards,
> Huang, Ying