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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1783949800; 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=/fZYAXoSj6PRbx/ktnt4V+LdEYD8kBj8ni3o5E8GfhY=; b=MBfD/6cErRByPlmJ5XfZhrPzxJnt1aKQA82EgiYHbsKk/0QzsI6E57BTeupqWiHQxIZoaw AcMkOF+iZIvz4ndKBL5yZeW3au6BToSMJ1u46ACcnz3QwIoQ737Yb/0mXPzrBj0TQwyijX EIiNG25EZcZGc/ZQWT7U+fQI9ymQRpI= 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, npache@redhat.com, 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 v4 03/11] mm: handle PMD swap entries in fork path Date: Mon, 13 Jul 2026 06:35:38 -0700 Message-ID: <20260713133613.2707815-4-usama.arif@linux.dev> In-Reply-To: <20260713133613.2707815-1-usama.arif@linux.dev> References: <20260713133613.2707815-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT Teach copy_huge_pmd()/copy_huge_non_present_pmd() about swap entries, mirroring copy_nonpresent_pte(). swap_dup_entry_direct() gains a nr parameter (and is renamed to swap_dup_entries_direct()) so it can duplicate a contiguous range of swap slots in one call, matching the existing swap_put_entries_direct(entry, nr) API. Existing callers pass 1. swap_retry_table_alloc() likewise gains a nr parameter so the outer retry knows how many slots the caller was trying to duplicate. The underlying swap_extend_table_alloc() now scans every slot in [ci_off, ci_off + nr) to confirm that at least one still needs the per-cluster extend table before committing an allocation. copy_huge_non_present_pmd() "copies" PMD swap entries during fork instead of splitting, preserving the THP. This mirrors copy_nonpresent_pte() which duplicates the swap slot refcount, clears the exclusive bit on the source, and adds the destination mm to mmlist. If swap_dup_entries_direct() fails (GFP_ATOMIC table alloc), copy_huge_pmd() retries after swap_retry_table_alloc(entry, HPAGE_PMD_NR, GFP_KERNEL), matching the PTE retry in copy_pte_range(). The PMD is stable across the retry because dup_mmap() holds write mmap_lock on both mm_structs. Signed-off-by: Usama Arif --- include/linux/swap.h | 4 ++-- mm/huge_memory.c | 54 ++++++++++++++++++++++++++++++++++++++------ mm/memory.c | 4 ++-- mm/swap.h | 5 ++-- mm/swapfile.c | 41 +++++++++++++++++++++------------ 5 files changed, 81 insertions(+), 27 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index 4427c7aa16dc..de1c3a897184 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -388,7 +388,7 @@ sector_t swap_folio_sector(struct folio *folio); * All entries must be allocated by folio_alloc_swap(). And they must have * a swap count > 1. See comments of folio_*_swap helpers for more info. */ -int swap_dup_entry_direct(swp_entry_t entry); +int swap_dup_entries_direct(swp_entry_t entry, int nr); void swap_put_entries_direct(swp_entry_t entry, int nr); /* @@ -432,7 +432,7 @@ static inline void free_swap_cache(struct folio *folio) { } -static inline int swap_dup_entry_direct(swp_entry_t ent) +static inline int swap_dup_entries_direct(swp_entry_t ent, int nr) { return 0; } diff --git a/mm/huge_memory.c b/mm/huge_memory.c index e602cf2b48db..d16cdf684b41 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1805,7 +1805,7 @@ bool touch_pmd(struct vm_area_struct *vma, unsigned long addr, return false; } -static void copy_huge_non_present_pmd( +static int copy_huge_non_present_pmd( struct mm_struct *dst_mm, struct mm_struct *src_mm, pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr, struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, @@ -1851,14 +1851,35 @@ static void copy_huge_non_present_pmd( */ folio_try_dup_anon_rmap_pmd(src_folio, &src_folio->page, dst_vma, src_vma); + } else if (softleaf_is_swap(entry)) { + int err; + + /* + * PMD swap entry: duplicate swap references and clear + * exclusive on source, matching copy_nonpresent_pte(). + */ + err = swap_dup_entries_direct(entry, HPAGE_PMD_NR); + if (err < 0) + return err; + + mm_prepare_for_swap_entries(dst_mm); + + if (pmd_swp_exclusive(pmd)) { + pmd = pmd_swp_clear_exclusive(pmd); + set_pmd_at(src_mm, addr, src_pmd, pmd); + } } - add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR); + if (softleaf_is_swap(entry)) + add_mm_counter(dst_mm, MM_SWAPENTS, HPAGE_PMD_NR); + else + add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR); mm_inc_nr_ptes(dst_mm); pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable); if (!userfaultfd_protected(dst_vma)) pmd = pmd_swp_clear_uffd(pmd); set_pmd_at(dst_mm, addr, dst_pmd, pmd); + return 0; } int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, @@ -1899,6 +1920,7 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, if (unlikely(!pgtable)) goto out; +retry: dst_ptl = pmd_lock(dst_mm, dst_pmd); src_ptl = pmd_lockptr(src_mm, src_pmd); spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); @@ -1906,11 +1928,29 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, ret = -EAGAIN; pmd = *src_pmd; - if (unlikely(thp_migration_supported() && - pmd_is_valid_softleaf(pmd))) { - copy_huge_non_present_pmd(dst_mm, src_mm, dst_pmd, src_pmd, addr, - dst_vma, src_vma, pmd, pgtable); - ret = 0; + if (unlikely(pmd_is_valid_softleaf(pmd))) { + ret = copy_huge_non_present_pmd(dst_mm, src_mm, dst_pmd, src_pmd, + addr, dst_vma, src_vma, pmd, + pgtable); + if (ret) { + spin_unlock(src_ptl); + spin_unlock(dst_ptl); + /* + * For PMD swap entries -ENOMEM means the per-cluster + * swap-extend table couldn't be GFP_ATOMIC-allocated. + * try the GFP_KERNEL fallback once before giving up. + */ + if (ret == -ENOMEM) { + softleaf_t entry = softleaf_from_pmd(pmd); + + if (softleaf_is_swap(entry) && + !swap_retry_table_alloc(entry, HPAGE_PMD_NR, + GFP_KERNEL)) + goto retry; + } + pte_free(dst_mm, pgtable); + goto out; + } goto out_unlock; } diff --git a/mm/memory.c b/mm/memory.c index d5e87624f692..21ea76bb4c37 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1010,7 +1010,7 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, struct page *page; if (likely(softleaf_is_swap(entry))) { - if (swap_dup_entry_direct(entry) < 0) + if (swap_dup_entries_direct(entry, 1) < 0) return -EIO; mm_prepare_for_swap_entries(dst_mm); @@ -1425,7 +1425,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, if (ret == -EIO) { VM_WARN_ON_ONCE(!entry.val); - if (swap_retry_table_alloc(entry, GFP_KERNEL) < 0) { + if (swap_retry_table_alloc(entry, 1, GFP_KERNEL) < 0) { ret = -ENOMEM; goto out; } diff --git a/mm/swap.h b/mm/swap.h index b51ad3071a73..c524a7670ac0 100644 --- a/mm/swap.h +++ b/mm/swap.h @@ -206,7 +206,7 @@ static inline void swap_cluster_unlock_irq(struct swap_cluster_info *ci) spin_unlock_irq(&ci->lock); } -extern int swap_retry_table_alloc(swp_entry_t entry, gfp_t gfp); +extern int swap_retry_table_alloc(swp_entry_t entry, unsigned int nr, gfp_t gfp); /* * Below are the core routines for doing swap for a folio. @@ -431,7 +431,8 @@ static inline int swap_writeout(struct folio *folio, return 0; } -static inline int swap_retry_table_alloc(swp_entry_t entry, gfp_t gfp) +static inline int swap_retry_table_alloc(swp_entry_t entry, unsigned int nr, + gfp_t gfp) { return -EINVAL; } diff --git a/mm/swapfile.c b/mm/swapfile.c index 72952491e9cf..2dbe52e1dfe0 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -1456,9 +1456,11 @@ static bool swap_sync_discard(void) static int swap_extend_table_alloc(struct swap_info_struct *si, struct swap_cluster_info *ci, - unsigned int ci_off, gfp_t gfp) + unsigned int ci_off, unsigned int nr, + gfp_t gfp) { int count; + unsigned int i; void *table; table = kzalloc(sizeof(ci->extend_table[0]) * SWAPFILE_CLUSTER, gfp); @@ -1474,15 +1476,23 @@ static int swap_extend_table_alloc(struct swap_info_struct *si, */ if (!cluster_table_is_alloced(ci)) goto out_free; - count = swp_tb_get_count(__swap_table_get(ci, ci_off)); - if (count < (SWP_TB_COUNT_MAX - 1)) - goto out_free; if (ci->extend_table) goto out_free; - - ci->extend_table = table; - spin_unlock(&ci->lock); - return 0; + /* + * The caller may not know which slot in [ci_off, ci_off + nr) hit + * SWP_TB_COUNT_MAX - 1 (e.g., the PMD fork retry only holds the + * range base after an atomic-alloc failure at a subslot). Confirm + * at least one slot in the range still needs the extend table + * before committing the allocation. + */ + for (i = 0; i < nr; i++) { + count = swp_tb_get_count(__swap_table_get(ci, ci_off + i)); + if (count >= (SWP_TB_COUNT_MAX - 1)) { + ci->extend_table = table; + spin_unlock(&ci->lock); + return 0; + } + } out_free: spin_unlock(&ci->lock); @@ -1490,7 +1500,7 @@ static int swap_extend_table_alloc(struct swap_info_struct *si, return 0; } -int swap_retry_table_alloc(swp_entry_t entry, gfp_t gfp) +int swap_retry_table_alloc(swp_entry_t entry, unsigned int nr, gfp_t gfp) { int ret; struct swap_info_struct *si; @@ -1502,7 +1512,8 @@ int swap_retry_table_alloc(swp_entry_t entry, gfp_t gfp) return 0; ci = __swap_offset_to_cluster(si, offset); - ret = swap_extend_table_alloc(si, ci, swp_cluster_offset(entry), gfp); + ret = swap_extend_table_alloc(si, ci, swp_cluster_offset(entry), nr, + gfp); put_swap_device(si); return ret; @@ -1703,7 +1714,8 @@ static int swap_dup_entries_cluster(struct swap_info_struct *si, if (unlikely(err)) { if (err == -ENOMEM) { spin_unlock(&ci->lock); - err = swap_extend_table_alloc(si, ci, ci_off, GFP_ATOMIC); + err = swap_extend_table_alloc(si, ci, ci_off, 1, + GFP_ATOMIC); spin_lock(&ci->lock); if (!err) goto restart; @@ -3904,8 +3916,9 @@ void si_swapinfo(struct sysinfo *val) } /* - * swap_dup_entry_direct() - Increase reference count of a swap entry by one. + * swap_dup_entries_direct() - Increase reference count of swap entries by one. * @entry: first swap entry from which we want to increase the refcount. + * @nr: number of contiguous swap entries to duplicate. * * Returns 0 for success, or -ENOMEM if the extend table is required * but could not be atomically allocated. Returns -EINVAL if the swap @@ -3917,7 +3930,7 @@ void si_swapinfo(struct sysinfo *val) * Also the swap entry must have a count >= 1. Otherwise folio_dup_swap should * be used. */ -int swap_dup_entry_direct(swp_entry_t entry) +int swap_dup_entries_direct(swp_entry_t entry, int nr) { struct swap_info_struct *si; @@ -3934,7 +3947,7 @@ int swap_dup_entry_direct(swp_entry_t entry) */ VM_WARN_ON_ONCE(!swap_entry_swapped(si, entry)); - return swap_dup_entries_cluster(si, swp_offset(entry), 1); + return swap_dup_entries_cluster(si, swp_offset(entry), nr); } #if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP) -- 2.53.0-Meta