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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id x18-20020a258592000000b00ba88763e5b5sm2667181ybk.2.2023.05.28.23.25.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 28 May 2023 23:25:19 -0700 (PDT) Date: Sun, 28 May 2023 23:25:15 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: Andrew Morton cc: Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , Matthew Wilcox , David Hildenbrand , Suren Baghdasaryan , Qi Zheng , Yang Shi , Mel Gorman , Peter Xu , Peter Zijlstra , Will Deacon , Yu Zhao , Alistair Popple , Ralph Campbell , Ira Weiny , Steven Price , SeongJae Park , Naoya Horiguchi , Christophe Leroy , Zack Rusin , Jason Gunthorpe , Axel Rasmussen , Anshuman Khandual , Pasha Tatashin , Miaohe Lin , Minchan Kim , Christoph Hellwig , Song Liu , Thomas Hellstrom , Russell King , "David S. Miller" , Michael Ellerman , "Aneesh Kumar K.V" , Heiko Carstens , Christian Borntraeger , Claudio Imbrenda , Alexander Gordeev , Jann Horn , linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH 09/12] mm/khugepaged: retract_page_tables() without mmap or vma lock In-Reply-To: <35e983f5-7ed3-b310-d949-9ae8b130cdab@google.com> Message-ID: <2e9996fa-d238-e7c-1194-834a2bd1f60@google.com> References: <35e983f5-7ed3-b310-d949-9ae8b130cdab@google.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Simplify shmem and file THP collapse's retract_page_tables(), and relax its locking: to improve its success rate and to lessen impact on others. Instead of its MADV_COLLAPSE case doing set_huge_pmd() at target_addr of target_mm, leave that part of the work to madvise_collapse() calling collapse_pte_mapped_thp() afterwards: just adjust collapse_file()'s result code to arrange for that. That spares retract_page_tables() four arguments; and since it will be successful in retracting all of the page tables expected of it, no need to track and return a result code itself. It needs i_mmap_lock_read(mapping) for traversing the vma interval tree, but it does not need i_mmap_lock_write() for that: page_vma_mapped_walk() allows for pte_offset_map_lock() etc to fail, and uses pmd_lock() for THPs. retract_page_tables() just needs to use those same spinlocks to exclude it briefly, while transitioning pmd from page table to none: so restore its use of pmd_lock() inside of which pte lock is nested. Users of pte_offset_map_lock() etc all now allow for them to fail: so retract_page_tables() now has no use for mmap_write_trylock() or vma_try_start_write(). In common with rmap and page_vma_mapped_walk(), it does not even need the mmap_read_lock(). But those users do expect the page table to remain a good page table, until they unlock and rcu_read_unlock(): so the page table cannot be freed immediately, but rather by the recently added pte_free_defer(). retract_page_tables() can be enhanced to replace_page_tables(), which inserts the final huge pmd without mmap lock: going through an invalid state instead of pmd_none() followed by fault. But that does raise some questions, and requires a more complicated pte_free_defer() for powerpc (when its arch_needs_pgtable_deposit() for shmem and file THPs). Leave that enhancement to a later release. Signed-off-by: Hugh Dickins --- mm/khugepaged.c | 169 +++++++++++++++++------------------------------- 1 file changed, 60 insertions(+), 109 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 1083f0e38a07..4fd408154692 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1617,9 +1617,8 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, uns= igned long addr, break; case SCAN_PMD_NONE: /* - * In MADV_COLLAPSE path, possible race with khugepaged where - * all pte entries have been removed and pmd cleared. If so, - * skip all the pte checks and just update the pmd mapping. + * All pte entries have been removed and pmd cleared. + * Skip all the pte checks and just update the pmd mapping. */ goto maybe_install_pmd; default: @@ -1748,123 +1747,73 @@ static void khugepaged_collapse_pte_mapped_thps(st= ruct khugepaged_mm_slot *mm_sl mmap_write_unlock(mm); } =20 -static int retract_page_tables(struct address_space *mapping, pgoff_t pgof= f, - struct mm_struct *target_mm, - unsigned long target_addr, struct page *hpage, - struct collapse_control *cc) +static void retract_page_tables(struct address_space *mapping, pgoff_t pgo= ff) { struct vm_area_struct *vma; - int target_result =3D SCAN_FAIL; =20 - i_mmap_lock_write(mapping); + i_mmap_lock_read(mapping); vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) { - int result =3D SCAN_FAIL; - struct mm_struct *mm =3D NULL; - unsigned long addr =3D 0; - pmd_t *pmd; - bool is_target =3D false; + struct mm_struct *mm; + unsigned long addr; + pmd_t *pmd, pgt_pmd; + spinlock_t *pml; + spinlock_t *ptl; =20 /* * Check vma->anon_vma to exclude MAP_PRIVATE mappings that - * got written to. These VMAs are likely not worth investing - * mmap_write_lock(mm) as PMD-mapping is likely to be split - * later. + * got written to. These VMAs are likely not worth removing + * page tables from, as PMD-mapping is likely to be split later. * - * Note that vma->anon_vma check is racy: it can be set up after - * the check but before we took mmap_lock by the fault path. - * But page lock would prevent establishing any new ptes of the - * page, so we are safe. - * - * An alternative would be drop the check, but check that page - * table is clear before calling pmdp_collapse_flush() under - * ptl. It has higher chance to recover THP for the VMA, but - * has higher cost too. It would also probably require locking - * the anon_vma. + * Note that vma->anon_vma check is racy: it can be set after + * the check, but page locks (with XA_RETRY_ENTRYs in holes) + * prevented establishing new ptes of the page. So we are safe + * to remove page table below, without even checking it's empty. */ - if (READ_ONCE(vma->anon_vma)) { - result =3D SCAN_PAGE_ANON; - goto next; - } + if (READ_ONCE(vma->anon_vma)) + continue; + addr =3D vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT); if (addr & ~HPAGE_PMD_MASK || - vma->vm_end < addr + HPAGE_PMD_SIZE) { - result =3D SCAN_VMA_CHECK; - goto next; - } - mm =3D vma->vm_mm; - is_target =3D mm =3D=3D target_mm && addr =3D=3D target_addr; - result =3D find_pmd_or_thp_or_none(mm, addr, &pmd); - if (result !=3D SCAN_SUCCEED) - goto next; - /* - * We need exclusive mmap_lock to retract page table. - * - * We use trylock due to lock inversion: we need to acquire - * mmap_lock while holding page lock. Fault path does it in - * reverse order. Trylock is a way to avoid deadlock. - * - * Also, it's not MADV_COLLAPSE's job to collapse other - * mappings - let khugepaged take care of them later. - */ - result =3D SCAN_PTE_MAPPED_HUGEPAGE; - if ((cc->is_khugepaged || is_target) && - mmap_write_trylock(mm)) { - /* trylock for the same lock inversion as above */ - if (!vma_try_start_write(vma)) - goto unlock_next; - - /* - * Re-check whether we have an ->anon_vma, because - * collapse_and_free_pmd() requires that either no - * ->anon_vma exists or the anon_vma is locked. - * We already checked ->anon_vma above, but that check - * is racy because ->anon_vma can be populated under the - * mmap lock in read mode. - */ - if (vma->anon_vma) { - result =3D SCAN_PAGE_ANON; - goto unlock_next; - } - /* - * When a vma is registered with uffd-wp, we can't - * recycle the pmd pgtable because there can be pte - * markers installed. Skip it only, so the rest mm/vma - * can still have the same file mapped hugely, however - * it'll always mapped in small page size for uffd-wp - * registered ranges. - */ - if (hpage_collapse_test_exit(mm)) { - result =3D SCAN_ANY_PROCESS; - goto unlock_next; - } - if (userfaultfd_wp(vma)) { - result =3D SCAN_PTE_UFFD_WP; - goto unlock_next; - } - collapse_and_free_pmd(mm, vma, addr, pmd); - if (!cc->is_khugepaged && is_target) - result =3D set_huge_pmd(vma, addr, pmd, hpage); - else - result =3D SCAN_SUCCEED; - -unlock_next: - mmap_write_unlock(mm); - goto next; - } - /* - * Calling context will handle target mm/addr. Otherwise, let - * khugepaged try again later. - */ - if (!is_target) { - khugepaged_add_pte_mapped_thp(mm, addr); + vma->vm_end < addr + HPAGE_PMD_SIZE) continue; - } -next: - if (is_target) - target_result =3D result; + + mm =3D vma->vm_mm; + if (find_pmd_or_thp_or_none(mm, addr, &pmd) !=3D SCAN_SUCCEED) + continue; + + if (hpage_collapse_test_exit(mm)) + continue; + /* + * When a vma is registered with uffd-wp, we cannot recycle + * the page table because there may be pte markers installed. + * Other vmas can still have the same file mapped hugely, but + * skip this one: it will always be mapped in small page size + * for uffd-wp registered ranges. + * + * What if VM_UFFD_WP is set a moment after this check? No + * problem, huge page lock is still held, stopping new mappings + * of page which might then get replaced by pte markers: only + * existing markers need to be protected here. (We could check + * after getting ptl below, but this comment distracting there!) + */ + if (userfaultfd_wp(vma)) + continue; + + /* Huge page lock is still held, so page table must be empty */ + pml =3D pmd_lock(mm, pmd); + ptl =3D pte_lockptr(mm, pmd); + if (ptl !=3D pml) + spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); + pgt_pmd =3D pmdp_collapse_flush(vma, addr, pmd); + if (ptl !=3D pml) + spin_unlock(ptl); + spin_unlock(pml); + + mm_dec_nr_ptes(mm); + page_table_check_pte_clear_range(mm, addr, pgt_pmd); + pte_free_defer(mm, pmd_pgtable(pgt_pmd)); } - i_mmap_unlock_write(mapping); - return target_result; + i_mmap_unlock_read(mapping); } =20 /** @@ -2261,9 +2210,11 @@ static int collapse_file(struct mm_struct *mm, unsig= ned long addr, =20 /* * Remove pte page tables, so we can re-fault the page as huge. + * If MADV_COLLAPSE, adjust result to call collapse_pte_mapped_thp(). */ - result =3D retract_page_tables(mapping, start, mm, addr, hpage, - cc); + retract_page_tables(mapping, start); + if (cc && !cc->is_khugepaged) + result =3D SCAN_PTE_MAPPED_HUGEPAGE; unlock_page(hpage); =20 /* --=20 2.35.3