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

Ryan Roberts
2024-03-12T09:41:00Z

On 12/03/2024 07:51, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@arm.com> 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@arm.com>
>> ---
>> 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