[PATCH mmotm] mm: unmap_mapping_range_tree() with i_mmap_rwsem shared

[Hugh Dickins]

Revert 48ec833b7851 ("Revert "mm/memory.c: share the i_mmap_rwsem"")
to reinstate c8475d144abb ("mm/memory.c: share the i_mmap_rwsem"): the
unmap_mapping_range family of functions do the unmapping of user pages
(ultimately via zap_page_range_single) without modifying the interval
tree itself, and unmapping races are necessarily guarded by page table
lock, thus the i_mmap_rwsem should be shared in unmap_mapping_pages()
and unmap_mapping_folio().

Commit 48ec833b7851 was intended as a short-term measure, allowing the
other shared lock changes into 3.19 final, before investigating three
trinity crashes, one of which had been bisected to commit c8475d144ab:

[1] https://lkml.org/lkml/2014/11/14/342
https://lore.kernel.org/lkml/5466142C.60100@xxxxxxxxxx/
[2] https://lkml.org/lkml/2014/12/22/213
https://lore.kernel.org/lkml/549832E2.8060609@xxxxxxxxxx/
[3] https://lkml.org/lkml/2014/12/9/741
https://lore.kernel.org/lkml/5487ACC5.1010002@xxxxxxxxxx/

Two of those were Bad page states: free_pages_prepare() found PG_mlocked
still set - almost certain to have been fixed by 4.4 commit b87537d9e2fe
("mm: rmap use pte lock not mmap_sem to set PageMlocked"). The NULL deref
on rwsem in [2]: unclear, only happened once, not bisected to c8475d144ab.

No change to the i_mmap_lock_write() around __unmap_hugepage_range_final()
in unmap_single_vma(): IIRC that's a special usage, helping to serialize
hugetlbfs page table sharing, not to be dabbled with lightly.  No change
to other uses of i_mmap_lock_write() by hugetlbfs.

I am not aware of any significant gains from the concurrency allowed by
this commit: it is submitted more to resolve an ancient misunderstanding.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 mm/memory.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3388,11 +3388,11 @@ void unmap_mapping_folio(struct folio *folio)
 	details.even_cows = false;
 	details.single_folio = folio;
 
-	i_mmap_lock_write(mapping);
+	i_mmap_lock_read(mapping);
 	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
 		unmap_mapping_range_tree(&mapping->i_mmap, first_index,
 					 last_index, &details);
-	i_mmap_unlock_write(mapping);
+	i_mmap_unlock_read(mapping);
 }
 
 /**
@@ -3418,11 +3418,11 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
 	if (last_index < first_index)
 		last_index = ULONG_MAX;
 
-	i_mmap_lock_write(mapping);
+	i_mmap_lock_read(mapping);
 	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
 		unmap_mapping_range_tree(&mapping->i_mmap, first_index,
 					 last_index, &details);
-	i_mmap_unlock_write(mapping);
+	i_mmap_unlock_read(mapping);
 }
 EXPORT_SYMBOL_GPL(unmap_mapping_pages);