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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784733763; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=usl6a5PLOpjeUD/D0yu6fB4iAln4bu/tH3LQhZj4L6o=; b=l0YweB/3vCYkJKFlc92L9HxZEN0qV4R3b+hOtmudk4D0E2Ism5BuVtRM4ltzbUuh8aJWIC d5+eHJ4qidogUr89Rvj1zaf7kFJZE/lEf77HQPIiUk3Ap11g+kI5WBfRFJiyCd8oGKTZfX iS6o3VrQx2EYXGjV8FLp4T6ygug+i60= From: Usama Arif To: Andrew Morton , david@kernel.org, chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org Cc: ying.huang@linux.alibaba.com, Baoquan He , willy@infradead.org, youngjun.park@lge.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, alex@ghiti.fr, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, Nico Pache , Liam R. Howlett , ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, kernel-team@meta.com, Usama Arif Subject: [PATCH v5 10/11] mm: install PMD swap entries on swap-out Date: Wed, 22 Jul 2026 08:19:41 -0700 Message-ID: <20260722152043.2273289-11-usama.arif@linux.dev> In-Reply-To: <20260722152043.2273289-1-usama.arif@linux.dev> References: <20260722152043.2273289-1-usama.arif@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 2F2C8140013 X-Stat-Signature: 3mdwzei6n7qxhseyfdm4jnfce56aph88 X-HE-Tag: 1784733765-724543 X-HE-Meta: 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 GmmufMYl yYt2Bz2CXQcj6deDUSUUyR4jcfegX5xChgc7EZmhIhN4+qhUaKZEkBKp0BEMR4VWvQ9U4ks87ZYCb6O8uMjMfrXcDUA1G/X8yx4fHCTzvWxKqJywMiPR4k/8o/F5oTQiHbGMCpejHlfc+9JqOQcGN77KhoJgfA+yq5tbVSxWwF4/o1PYhweldTHBJTgFLr55nP/1P/21wRJZAZu0= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Reclaim today splits a PMD-mapped anonymous THP into 512 PTE swap entries before unmap, losing the huge mapping across the swap round-trip and forcing khugepaged to rebuild it later. The contiguous swap range was already secured when the folio was added to the swap cache (a non-contiguous allocation would have split the folio earlier), so the PMD can be replaced by a single PMD-level swap entry instead. This patch mirrors the existing PTE swap-out path at PMD granularity: - shrink_folio_list() drops TTU_SPLIT_HUGE_PMD for PMD-mappable swapcache folios. zswap is handled by the PMD swap-in users: if any covered slot currently has a zswap entry, they split the PMD swap entry and fall back to the per-PTE path. - try_to_unmap_one() now has a PMD branch that calls set_pmd_swap_entry() and adjusts MM_ANONPAGES / MM_SWAPENTS by HPAGE_PMD_NR before walk_done. TTU_SPLIT_HUGE_PMD remains the fallback. - set_pmd_swap_entry() is the installer. Mirroring the PTE swap-out sequence at PMD granularity, it clears the present mapping (keeping the original for rollback), bumps the swap_map refcount for the folio's 512 slots, transfers the exclusive state in the swap entry, propagates the dirty bit to the folio so writeback is not lost, and installs a swap PMD that preserves the original soft-dirty / uffd-wp / exclusive bits. Any failing step rolls back the present mapping. The swap entry value matches what 512 PTE swap entries would encode, so swap_map refcounting is unchanged: each of the 512 slots carries a count of 1, released individually on later split or together on swap-in. Add thp_swpout_pmd to count each PMD mapping replaced by a PMD-level swap entry. Unlike the folio-level thp_swpout counter, a fork-shared THP can increment this counter once for each mapping; document that distinction. Signed-off-by: Usama Arif --- Documentation/admin-guide/mm/transhuge.rst | 5 ++ include/linux/huge_mm.h | 2 + include/linux/vm_event_item.h | 1 + mm/huge_memory.c | 80 ++++++++++++++++++++++ mm/rmap.c | 19 +++++ mm/vmscan.c | 9 ++- mm/vmstat.c | 1 + 7 files changed, 116 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst index 16f37135ed80..b421d7982db6 100644 --- a/Documentation/admin-guide/mm/transhuge.rst +++ b/Documentation/admin-guide/mm/transhuge.rst @@ -630,6 +630,11 @@ thp_swpout is incremented every time a huge page is swapout in one piece without splitting. +thp_swpout_pmd + is incremented every time a PMD mapping is replaced by a PMD-level + swap entry. A fork-shared THP can increment this counter once for each + PMD mapping that is swapped out. + thp_swpout_fallback is incremented if a huge page has to be split before swapout. Usually because failed to allocate some continuous swap space diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index e7107e0991ad..41cf643a3f55 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -554,6 +554,8 @@ vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf); #ifdef CONFIG_THP_SWAP vm_fault_t do_huge_pmd_swap_page(struct vm_fault *vmf); +int set_pmd_swap_entry(struct page_vma_mapped_walk *pvmw, + struct folio *folio); #else static inline vm_fault_t do_huge_pmd_swap_page(struct vm_fault *vmf) { diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h index 2628ccda076a..f8fd4e13698c 100644 --- a/include/linux/vm_event_item.h +++ b/include/linux/vm_event_item.h @@ -108,6 +108,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT, THP_ZERO_PAGE_ALLOC_FAILED, THP_SWPOUT, THP_SWPOUT_FALLBACK, + THP_SWPOUT_PMD, #endif #ifdef CONFIG_BALLOON BALLOON_INFLATE, diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 6ef56936ced4..c014631e5a26 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -5561,3 +5561,83 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new) trace_remove_migration_pmd(address, pmd_val(pmde)); } #endif + +#ifdef CONFIG_THP_SWAP +/** + * set_pmd_swap_entry() - Replace a PMD mapping with a PMD-level swap entry. + * @pvmw: Page vma mapped walk context, must have pvmw->pmd set and + * pvmw->pte NULL (i.e. PMD-mapped). + * @folio: The folio being swapped out. Must be in the swap cache. + * + * This installs a PMD-level swap entry in place of a present PMD mapping, + * avoiding the need to split the PMD into PTE-level swap entries. + * + * Return: 0 on success, negative error code on failure. + */ +int set_pmd_swap_entry(struct page_vma_mapped_walk *pvmw, + struct folio *folio) +{ + struct vm_area_struct *vma = pvmw->vma; + struct mm_struct *mm = vma->vm_mm; + unsigned long address = pvmw->address; + unsigned long haddr = address & HPAGE_PMD_MASK; + struct page *page = folio_page(folio, 0); + bool anon_exclusive; + pmd_t pmdval; + swp_entry_t entry; + pmd_t pmdswp; + + if (!(pvmw->pmd && !pvmw->pte)) + return 0; + + VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio); + VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio); + + if (unlikely(folio_test_swapbacked(folio) != + folio_test_swapcache(folio))) { + WARN_ON_ONCE(1); + return -EBUSY; + } + + flush_cache_range(vma, haddr, haddr + HPAGE_PMD_SIZE); + + pmdval = pmdp_invalidate(vma, haddr, pvmw->pmd); + + /* Update high watermark before we lower rss */ + update_hiwater_rss(mm); + + if (folio_dup_swap(folio, NULL) < 0) { + set_pmd_at(mm, haddr, pvmw->pmd, pmdval); + return -ENOMEM; + } + + /* See folio_try_share_anon_rmap_pmd(): invalidate PMD first. */ + anon_exclusive = PageAnonExclusive(page); + if (anon_exclusive && folio_try_share_anon_rmap_pmd(folio, page)) { + folio_put_swap(folio, NULL); + set_pmd_at(mm, haddr, pvmw->pmd, pmdval); + return -EBUSY; + } + + mm_prepare_for_swap_entries(mm); + + if (pmd_dirty(pmdval)) + folio_mark_dirty(folio); + + entry = folio->swap; + pmdswp = softleaf_to_pmd(entry); + if (pmd_soft_dirty(pmdval)) + pmdswp = pmd_swp_mksoft_dirty(pmdswp); + if (pmd_uffd(pmdval)) + pmdswp = pmd_swp_mkuffd(pmdswp); + if (anon_exclusive) + pmdswp = pmd_swp_mkexclusive(pmdswp); + set_pmd_at(mm, haddr, pvmw->pmd, pmdswp); + + folio_remove_rmap_pmd(folio, page, vma); + folio_put(folio); + + count_vm_event(THP_SWPOUT_PMD); + return 0; +} +#endif /* CONFIG_THP_SWAP */ diff --git a/mm/rmap.c b/mm/rmap.c index b7ead3e9f064..4574f7b969b6 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2282,6 +2282,25 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, goto walk_abort; } +#ifdef CONFIG_THP_SWAP + /* + * If the folio is in the swap cache and we're not + * asked to split, install a PMD-level swap entry. + */ + if (!(flags & TTU_SPLIT_HUGE_PMD) && + folio_test_anon(folio) && + folio_test_swapcache(folio)) { + if (set_pmd_swap_entry(&pvmw, folio)) + goto walk_abort; + + add_mm_counter(mm, MM_ANONPAGES, + -HPAGE_PMD_NR); + add_mm_counter(mm, MM_SWAPENTS, + HPAGE_PMD_NR); + goto walk_done; + } +#endif + if (flags & TTU_SPLIT_HUGE_PMD) { /* * We temporarily have to drop the PTL and diff --git a/mm/vmscan.c b/mm/vmscan.c index 9b8ee902f972..83bb34403e42 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1333,7 +1333,14 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, enum ttu_flags flags = TTU_BATCH_FLUSH; bool was_swapbacked = folio_test_swapbacked(folio); - if (folio_test_pmd_mappable(folio)) + /* + * With THP_SWAP, PMD-mappable folios already in the + * swap cache can be unmapped with a PMD-level swap + * entry, avoiding the cost of splitting the PMD. + */ + if (folio_test_pmd_mappable(folio) && + !(IS_ENABLED(CONFIG_THP_SWAP) && + folio_test_swapcache(folio))) flags |= TTU_SPLIT_HUGE_PMD; /* * Without TTU_SYNC, try_to_unmap will only begin to diff --git a/mm/vmstat.c b/mm/vmstat.c index 4e26e5fd6666..68e6efc0dabd 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -1422,6 +1422,7 @@ const char * const vmstat_text[] = { [I(THP_ZERO_PAGE_ALLOC_FAILED)] = "thp_zero_page_alloc_failed", [I(THP_SWPOUT)] = "thp_swpout", [I(THP_SWPOUT_FALLBACK)] = "thp_swpout_fallback", + [I(THP_SWPOUT_PMD)] = "thp_swpout_pmd", #endif #ifdef CONFIG_BALLOON [I(BALLOON_INFLATE)] = "balloon_inflate", -- 2.53.0-Meta