From 610d06576737f5401647a4aab46558c1114898fb Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 11 Jul 2023 21:46:23 -0700 Subject: [PATCH] mm/pgtable: notes on pte_offset_map[_lock]() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Add a block of comments on pte_offset_map_lock(), pte_offset_map() and pte_offset_map_nolock() to mm/pgtable-generic.c, to help explain them. Link: https://lkml.kernel.org/r/b791c3b0-25c6-a263-d785-d564344eb644@google.com Signed-off-by: Hugh Dickins Cc: Alexander Gordeev Cc: Alistair Popple Cc: Aneesh Kumar K.V Cc: Anshuman Khandual Cc: Axel Rasmussen Cc: Christian Borntraeger Cc: Christophe Leroy Cc: Christoph Hellwig Cc: Claudio Imbrenda Cc: David Hildenbrand Cc: "David S. Miller" Cc: Gerald Schaefer Cc: Heiko Carstens Cc: Huang, Ying Cc: Ira Weiny Cc: Jann Horn Cc: Jason Gunthorpe Cc: Kirill A. Shutemov Cc: Lorenzo Stoakes Cc: Matthew Wilcox (Oracle) Cc: Mel Gorman Cc: Miaohe Lin Cc: Michael Ellerman Cc: Mike Kravetz Cc: Mike Rapoport (IBM) Cc: Minchan Kim Cc: Naoya Horiguchi Cc: Pavel Tatashin Cc: Peter Xu Cc: Peter Zijlstra Cc: Qi Zheng Cc: Ralph Campbell Cc: Russell King Cc: SeongJae Park Cc: Song Liu Cc: Steven Price Cc: Suren Baghdasaryan Cc: Thomas Hellström Cc: Vasily Gorbik Cc: Vishal Moola (Oracle) Cc: Vlastimil Babka Cc: Will Deacon Cc: Yang Shi Cc: Yu Zhao Cc: Zack Rusin Cc: Zi Yan Signed-off-by: Andrew Morton --- mm/pgtable-generic.c | 44 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c index fa9d4d084291..4fcd959dcc4d 100644 --- a/mm/pgtable-generic.c +++ b/mm/pgtable-generic.c @@ -315,6 +315,50 @@ pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd, return pte; } +/* + * pte_offset_map_lock(mm, pmd, addr, ptlp), and its internal implementation + * __pte_offset_map_lock() below, is usually called with the pmd pointer for + * addr, reached by walking down the mm's pgd, p4d, pud for addr: either while + * holding mmap_lock or vma lock for read or for write; or in truncate or rmap + * context, while holding file's i_mmap_lock or anon_vma lock for read (or for + * write). In a few cases, it may be used with pmd pointing to a pmd_t already + * copied to or constructed on the stack. + * + * When successful, it returns the pte pointer for addr, with its page table + * kmapped if necessary (when CONFIG_HIGHPTE), and locked against concurrent + * modification by software, with a pointer to that spinlock in ptlp (in some + * configs mm->page_table_lock, in SPLIT_PTLOCK configs a spinlock in table's + * struct page). pte_unmap_unlock(pte, ptl) to unlock and unmap afterwards. + * + * But it is unsuccessful, returning NULL with *ptlp unchanged, if there is no + * page table at *pmd: if, for example, the page table has just been removed, + * or replaced by the huge pmd of a THP. (When successful, *pmd is rechecked + * after acquiring the ptlock, and retried internally if it changed: so that a + * page table can be safely removed or replaced by THP while holding its lock.) + * + * pte_offset_map(pmd, addr), and its internal helper __pte_offset_map() above, + * just returns the pte pointer for addr, its page table kmapped if necessary; + * or NULL if there is no page table at *pmd. It does not attempt to lock the + * page table, so cannot normally be used when the page table is to be updated, + * or when entries read must be stable. But it does take rcu_read_lock(): so + * that even when page table is racily removed, it remains a valid though empty + * and disconnected table. Until pte_unmap(pte) unmaps and rcu_read_unlock()s + * afterwards. + * + * pte_offset_map_nolock(mm, pmd, addr, ptlp), above, is like pte_offset_map(); + * but when successful, it also outputs a pointer to the spinlock in ptlp - as + * pte_offset_map_lock() does, but in this case without locking it. This helps + * the caller to avoid a later pte_lockptr(mm, *pmd), which might by that time + * act on a changed *pmd: pte_offset_map_nolock() provides the correct spinlock + * pointer for the page table that it returns. In principle, the caller should + * recheck *pmd once the lock is taken; in practice, no callsite needs that - + * either the mmap_lock for write, or pte_same() check on contents, is enough. + * + * Note that free_pgtables(), used after unmapping detached vmas, or when + * exiting the whole mm, does not take page table lock before freeing a page + * table, and may not use RCU at all: "outsiders" like khugepaged should avoid + * pte_offset_map() and co once the vma is detached from mm or mm_users is zero. + */ pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, unsigned long addr, spinlock_t **ptlp) {