From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: mm: unmap_mapping_range_tree() with i_mmap_rwsem shared
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.
Link: https://lkml.kernel.org/r/e4a5e356-6c87-47b2-3ce8-c2a95ae84e20@xxxxxxxxxx
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Sasha Levin <sashal@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memory.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
--- a/mm/memory.c~mm-unmap_mapping_range_tree-with-i_mmap_rwsem-shared
+++ a/mm/memory.c
@@ -3388,11 +3388,11 @@ void unmap_mapping_folio(struct folio *f
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_
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);
_
