mm/pgtable: add rcu_read_lock() and rcu_read_unlock()s

Patch series "mm: free retracted page table by RCU", v3.

Some mmap_lock avoidance i.e.  latency reduction.  Initially just for the
case of collapsing shmem or file pages to THPs: the usefulness of
MADV_COLLAPSE on shmem is being limited by that mmap_write_lock it
currently requires.

Likely to be relied upon later in other contexts e.g.  freeing of empty
page tables (but that's not work I'm doing).  mmap_write_lock avoidance
when collapsing to anon THPs?  Perhaps, but again that's not work I've
done: a quick attempt was not as easy as the shmem/file case.

These changes (though of course not these exact patches) have been in
Google's data centre kernel for three years now: we do rely upon them.


This patch (of 13):

Before putting them to use (several commits later), add rcu_read_lock() to
pte_offset_map(), and rcu_read_unlock() to pte_unmap().  Make this a
separate commit, since it risks exposing imbalances: prior commits have
fixed all the known imbalances, but we may find some have been missed.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Claudio Imbrenda <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Huang, Ying <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Qi Zheng <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Russell King <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Steven Price <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Thomas Hellström <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Vishal Moola (Oracle) <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: Zack Rusin <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

include/linux/pgtable.h

@@ -99,7 +99,7 @@ static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address)
 	((pte_t *)kmap_local_page(pmd_page(*(pmd))) + pte_index((address)))
 #define pte_unmap(pte)	do {	\
 	kunmap_local((pte));	\
-	/* rcu_read_unlock() to be added later */	\
+	rcu_read_unlock();	\
 } while (0)
 #else
 static inline pte_t *__pte_map(pmd_t *pmd, unsigned long address)
@@ -108,7 +108,7 @@ static inline pte_t *__pte_map(pmd_t *pmd, unsigned long address)
 }
 static inline void pte_unmap(pte_t *pte)
 {
-	/* rcu_read_unlock() to be added later */
+	rcu_read_unlock();
 }
 #endif
 

mm/pgtable-generic.c

@@ -236,7 +236,7 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
 {
 	pmd_t pmdval;
 
-	/* rcu_read_lock() to be added later */
+	rcu_read_lock();
 	pmdval = pmdp_get_lockless(pmd);
 	if (pmdvalp)
 		*pmdvalp = pmdval;
@@ -250,7 +250,7 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
 	}
 	return __pte_map(&pmdval, addr);
 nomap:
-	/* rcu_read_unlock() to be added later */
+	rcu_read_unlock();
 	return NULL;
 }