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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, qi.zheng@linux.dev, luizcap@redhat.com, kernel-team@meta.com, Usama Arif Subject: [PATCH v9 14/32] mm: handle PMD swap entries in fork path Date: Thu, 5 Nov 2026 10:38:58 -0800 Message-ID: <20261105183940.2236717-15-usama.arif@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261105183940.2236717-1-usama.arif@linux.dev> References: <20261105183940.2236717-1-usama.arif@linux.dev> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit copy_huge_pmd() only knows about migration and device-private PMDs, so a PMD swap entry would fall through to the present-PMD path and fork() would duplicate it without taking a reference on the slots it points at. Copy it the way copy_nonpresent_pte() copies a PTE swap entry: duplicate the swap references, clear the exclusive marker on the source, put the destination mm on mmlist, and account the child's slots to MM_SWAPENTS. The GFP_ATOMIC extend-table allocation inside the dup can fail. Report that as -EIO and let copy_pmd_range() retry with GFP_KERNEL, as copy_nonpresent_pte() and copy_pte_range() already do for a PTE swap entry. copy_huge_pmd() hands the entry back so the caller knows which range to allocate for. Only -ENOMEM is reported that way. The other failures mean the entry itself is bad, and swap_retry_table_alloc_nr() returns 0 for those, so collapsing them into -EIO as the PTE path does would spin in the caller's retry rather than failing the fork. While here, move the mm counter update into each entry-type arm, as the PTE version does, so the swap arm can account MM_SWAPENTS instead of MM_ANONPAGES. Signed-off-by: Usama Arif Reviewed-by: Zi Yan --- include/linux/huge_mm.h | 3 +- mm/huge_memory.c | 65 ++++++++++++++++++++++++++++++----------- mm/memory.c | 12 +++++++- 3 files changed, 61 insertions(+), 19 deletions(-) diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index 34307bda8071b..8597bc52fdc87 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -10,7 +10,8 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf); int copy_huge_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); + struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, + softleaf_t *entryp); bool huge_pmd_set_accessed(struct vm_fault *vmf); int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm, pud_t *dst_pud, pud_t *src_pud, unsigned long addr, diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 696813776e45d..acb540235e5a9 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1892,7 +1892,7 @@ 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, @@ -1900,18 +1900,44 @@ static void copy_huge_non_present_pmd( { softleaf_t entry = softleaf_from_pmd(pmd); struct folio *src_folio; + int err; VM_WARN_ON_ONCE(!pmd_is_valid_softleaf(pmd)); - if (softleaf_is_migration_write(entry) || - softleaf_is_migration_read_exclusive(entry)) { - entry = make_readable_migration_entry(swp_offset(entry)); - pmd = softleaf_to_pmd(entry); - if (pmd_swp_soft_dirty(*src_pmd)) - pmd = pmd_swp_mksoft_dirty(pmd); - if (pmd_swp_uffd(*src_pmd)) - pmd = pmd_swp_mkuffd(pmd); - set_pmd_at(src_mm, addr, src_pmd, pmd); + if (softleaf_is_swap(entry)) { + /* + * A PMD swap entry only exists under CONFIG_THP_SWAP, where + * SWAPFILE_CLUSTER == HPAGE_PMD_NR, and it is cluster aligned, + * so these HPAGE_PMD_NR slots are exactly one cluster - which + * is what swap_dup_entries_direct() requires. + */ + err = swap_dup_entries_direct(entry, HPAGE_PMD_NR); + /* + * Return -EIO on allocation failure so the caller can retry + * with GFP_KERNEL. Fail the fork on any other error. + */ + if (err) + return err == -ENOMEM ? -EIO : -ENOMEM; + + mm_prepare_for_swap_entries(dst_mm); + /* Mark the swap entry as shared. */ + 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_SWAPENTS, HPAGE_PMD_NR); + } else if (softleaf_is_migration(entry)) { + if (softleaf_is_migration_write(entry) || + softleaf_is_migration_read_exclusive(entry)) { + entry = make_readable_migration_entry(swp_offset(entry)); + pmd = softleaf_to_pmd(entry); + if (pmd_swp_soft_dirty(*src_pmd)) + pmd = pmd_swp_mksoft_dirty(pmd); + if (pmd_swp_uffd(*src_pmd)) + pmd = pmd_swp_mkuffd(pmd); + set_pmd_at(src_mm, addr, src_pmd, pmd); + } + add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR); } else if (softleaf_is_device_private(entry)) { /* * For device private entries, since there are no @@ -1938,19 +1964,21 @@ static void copy_huge_non_present_pmd( */ folio_try_dup_anon_rmap_pmd(src_folio, &src_folio->page, dst_vma, src_vma); + add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR); } - 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, pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr, - struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) + struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, + softleaf_t *entryp) { spinlock_t *dst_ptl, *src_ptl; struct page *src_page; @@ -1993,11 +2021,14 @@ 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) { + *entryp = softleaf_from_pmd(pmd); + pte_free(dst_mm, pgtable); + } goto out_unlock; } diff --git a/mm/memory.c b/mm/memory.c index 85bd1b08a1a2a..dedcec3cfca6e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1437,11 +1437,21 @@ copy_pmd_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, do { next = pmd_addr_end(addr, end); if (pmd_is_huge(*src_pmd)) { + softleaf_t entry = softleaf_mk_none(); int err; VM_BUG_ON_VMA(next-addr != HPAGE_PMD_SIZE, src_vma); +again: err = copy_huge_pmd(dst_mm, src_mm, dst_pmd, src_pmd, - addr, dst_vma, src_vma); + addr, dst_vma, src_vma, &entry); + if (err == -EIO) { + VM_WARN_ON_ONCE(!entry.val); + if (swap_retry_table_alloc_nr(entry, + HPAGE_PMD_NR, + GFP_KERNEL) < 0) + return -ENOMEM; + goto again; + } if (err == -ENOMEM) return -ENOMEM; if (!err) -- 2.53.0-Meta