From: Ryan Roberts ryan.roberts@arm.com
next inclusion category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/I9OCYO CVE: NA
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
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Patch series "Swap-out mTHP without splitting", v7.
This series adds support for swapping out multi-size THP (mTHP) without needing to first split the large folio via split_huge_page_to_list_to_order(). It closely follows the approach already used to swap-out PMD-sized THP.
There are a couple of reasons for swapping out mTHP without splitting:
- Performance: It is expensive to split a large folio and under extreme memory pressure some workloads regressed performance when using 64K mTHP vs 4K small folios because of this extra cost in the swap-out path. This series not only eliminates the regression but makes it faster to swap out 64K mTHP vs 4K small folios.
- Memory fragmentation avoidance: If we can avoid splitting a large folio memory is less likely to become fragmented, making it easier to re-allocate a large folio in future.
- Performance: Enables a separate series [7] to swap-in whole mTHPs, which means we won't lose the TLB-efficiency benefits of mTHP once the memory has been through a swap cycle.
I've done what I thought was the smallest change possible, and as a result, this approach is only employed when the swap is backed by a non-rotating block device (just as PMD-sized THP is supported today). Discussion against the RFC concluded that this is sufficient.
Performance Testing ===================
I've run some swap performance tests on Ampere Altra VM (arm64) with 8 CPUs. The VM is set up with a 35G block ram device as the swap device and the test is run from inside a memcg limited to 40G memory. I've then run `usemem` from vm-scalability with 70 processes, each allocating and writing 1G of memory. I've repeated everything 6 times and taken the mean performance improvement relative to 4K page baseline:
| alloc size | baseline | + this series | | | mm-unstable (~v6.9-rc1) | | |:-----------|------------------------:|------------------------:| | 4K Page | 0.0% | 1.3% | | 64K THP | -13.6% | 46.3% | | 2M THP | 91.4% | 89.6% |
So with this change, the 64K swap performance goes from a 14% regression to a 46% improvement. While 2M shows a small regression I'm confident that this is just noise.
[1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@arm.c... [2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@arm.c... [3] https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@arm.co... [4] https://lore.kernel.org/linux-mm/20240311150058.1122862-1-ryan.roberts@arm.c... [5] https://lore.kernel.org/linux-mm/20240327144537.4165578-1-ryan.roberts@arm.c... [6] https://lore.kernel.org/linux-mm/20240403114032.1162100-1-ryan.roberts@arm.c... [7] https://lore.kernel.org/linux-mm/20240304081348.197341-1-21cnbao@gmail.com/ [8] https://lore.kernel.org/linux-mm/CAGsJ_4yMOow27WDvN2q=E4HAtDd2PJ=OQ5Pj9DG+6F... [9] https://lore.kernel.org/linux-mm/579d5127-c763-4001-9625-4563a9316ac3@redhat...
This patch (of 7):
As preparation for supporting small-sized THP in the swap-out path, without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE, which, when present, always implies PMD-sized THP, which is the same as the cluster size.
The only use of the flag was to determine whether a swap entry refers to a single page or a PMD-sized THP in swap_page_trans_huge_swapped(). Instead of relying on the flag, we now pass in order, which originates from the folio's order. This allows the logic to work for folios of any order.
The one snag is that one of the swap_page_trans_huge_swapped() call sites does not have the folio. But it was only being called there to shortcut a call __try_to_reclaim_swap() in some cases. __try_to_reclaim_swap() gets the folio and (via some other functions) calls swap_page_trans_huge_swapped(). So I've removed the problematic call site and believe the new logic should be functionally equivalent.
That said, removing the fast path means that we will take a reference and trylock a large folio much more often, which we would like to avoid. The next patch will solve this.
Removing CLUSTER_FLAG_HUGE also means we can remove split_swap_cluster() which used to be called during folio splitting, since split_swap_cluster()'s only job was to remove the flag.
Link: https://lkml.kernel.org/r/20240408183946.2991168-1-ryan.roberts@arm.com Link: https://lkml.kernel.org/r/20240408183946.2991168-2-ryan.roberts@arm.com Signed-off-by: Ryan Roberts ryan.roberts@arm.com Reviewed-by: "Huang, Ying" ying.huang@intel.com Acked-by: Chris Li chrisl@kernel.org Acked-by: David Hildenbrand david@redhat.com Cc: Barry Song 21cnbao@gmail.com Cc: Gao Xiang xiang@kernel.org Cc: Kefeng Wang wangkefeng.wang@huawei.com Cc: Lance Yang ioworker0@gmail.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Michal Hocko mhocko@suse.com Cc: Yang Shi shy828301@gmail.com Cc: Yu Zhao yuzhao@google.com Cc: Barry Song v-songbaohua@oppo.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Liu Shixin liushixin2@huawei.com --- include/linux/swap.h | 10 ---------- mm/huge_memory.c | 3 --- mm/swapfile.c | 47 ++++++++------------------------------------ 3 files changed, 8 insertions(+), 52 deletions(-)
diff --git a/include/linux/swap.h b/include/linux/swap.h index cbcb767b79e3..4a2a8095d7ca 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -271,7 +271,6 @@ struct swap_cluster_info { }; #define CLUSTER_FLAG_FREE 1 /* This cluster is free */ #define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */ -#define CLUSTER_FLAG_HUGE 4 /* This cluster is backing a transparent huge page */
/* * We assign a cluster to each CPU, so each CPU can allocate swap entry from @@ -630,15 +629,6 @@ static inline int add_swap_extent(struct swap_info_struct *sis, } #endif /* CONFIG_SWAP */
-#ifdef CONFIG_THP_SWAP -extern int split_swap_cluster(swp_entry_t entry); -#else -static inline int split_swap_cluster(swp_entry_t entry) -{ - return 0; -} -#endif - #ifdef CONFIG_MEMCG static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg) { diff --git a/mm/huge_memory.c b/mm/huge_memory.c index a548caef1c2f..0c61e7c7c2c1 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2834,9 +2834,6 @@ static void __split_huge_page(struct page *page, struct list_head *list, shmem_uncharge(head->mapping->host, nr_dropped); remap_page(folio, nr);
- if (folio_test_swapcache(folio)) - split_swap_cluster(folio->swap); - /* * set page to its compound_head when split to non order-0 pages, so * we can skip unlocking it below, since PG_locked is transferred to diff --git a/mm/swapfile.c b/mm/swapfile.c index 961862f4aa57..25ff4f0dd2c2 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -342,18 +342,6 @@ static inline void cluster_set_null(struct swap_cluster_info *info) info->data = 0; }
-static inline bool cluster_is_huge(struct swap_cluster_info *info) -{ - if (IS_ENABLED(CONFIG_THP_SWAP)) - return info->flags & CLUSTER_FLAG_HUGE; - return false; -} - -static inline void cluster_clear_huge(struct swap_cluster_info *info) -{ - info->flags &= ~CLUSTER_FLAG_HUGE; -} - static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si, unsigned long offset) { @@ -1021,7 +1009,7 @@ static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot) offset = idx * SWAPFILE_CLUSTER; ci = lock_cluster(si, offset); alloc_cluster(si, idx); - cluster_set_count_flag(ci, SWAPFILE_CLUSTER, CLUSTER_FLAG_HUGE); + cluster_set_count(ci, SWAPFILE_CLUSTER);
memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER); unlock_cluster(ci); @@ -1449,7 +1437,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
ci = lock_cluster_or_swap_info(si, offset); if (size == SWAPFILE_CLUSTER) { - VM_BUG_ON(!cluster_is_huge(ci)); map = si->swap_map + offset; for (i = 0; i < SWAPFILE_CLUSTER; i++) { val = map[i]; @@ -1457,7 +1444,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry) if (val == SWAP_HAS_CACHE) free_entries++; } - cluster_clear_huge(ci); if (free_entries == SWAPFILE_CLUSTER) { unlock_cluster_or_swap_info(si, ci); spin_lock(&si->lock); @@ -1479,23 +1465,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry) unlock_cluster_or_swap_info(si, ci); }
-#ifdef CONFIG_THP_SWAP -int split_swap_cluster(swp_entry_t entry) -{ - struct swap_info_struct *si; - struct swap_cluster_info *ci; - unsigned long offset = swp_offset(entry); - - si = _swap_info_get(entry); - if (!si) - return -EBUSY; - ci = lock_cluster(si, offset); - cluster_clear_huge(ci); - unlock_cluster(ci); - return 0; -} -#endif - static int swp_entry_cmp(const void *ent1, const void *ent2) { const swp_entry_t *e1 = ent1, *e2 = ent2; @@ -1603,22 +1572,23 @@ int swp_swapcount(swp_entry_t entry) }
static bool swap_page_trans_huge_swapped(struct swap_info_struct *si, - swp_entry_t entry) + swp_entry_t entry, int order) { struct swap_cluster_info *ci; unsigned char *map = si->swap_map; + unsigned int nr_pages = 1 << order; unsigned long roffset = swp_offset(entry); - unsigned long offset = round_down(roffset, SWAPFILE_CLUSTER); + unsigned long offset = round_down(roffset, nr_pages); int i; bool ret = false;
ci = lock_cluster_or_swap_info(si, offset); - if (!ci || !cluster_is_huge(ci)) { + if (!ci || nr_pages == 1) { if (swap_count(map[roffset])) ret = true; goto unlock_out; } - for (i = 0; i < SWAPFILE_CLUSTER; i++) { + for (i = 0; i < nr_pages; i++) { if (swap_count(map[offset + i])) { ret = true; break; @@ -1640,7 +1610,7 @@ static bool folio_swapped(struct folio *folio) if (!IS_ENABLED(CONFIG_THP_SWAP) || likely(!folio_test_large(folio))) return swap_swapcount(si, entry) != 0;
- return swap_page_trans_huge_swapped(si, entry); + return swap_page_trans_huge_swapped(si, entry, folio_order(folio)); }
/** @@ -1706,8 +1676,7 @@ int free_swap_and_cache(swp_entry_t entry) }
count = __swap_entry_free(p, entry); - if (count == SWAP_HAS_CACHE && - !swap_page_trans_huge_swapped(p, entry)) + if (count == SWAP_HAS_CACHE) __try_to_reclaim_swap(p, swp_offset(entry), TTRS_UNMAPPED | TTRS_FULL); put_swap_device(p);