mm/pgtable: notes on pte_offset_map[_lock]()

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 <hughd@google.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Huang, Ying <ying.huang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: SeongJae Park <sj@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zack Rusin <zackr@vmware.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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)
 {