The following concurrency may cause the data read to be inconsistent with
the data on disk:
cpu1 cpu2
------------------------------|------------------------------
// Buffered write 2048 from 0
ext4_buffered_write_iter
generic_perform_write
copy_page_from_iter_atomic
ext4_da_write_end
ext4_da_do_write_end
block_write_end
__block_commit_write
folio_mark_uptodate
// Buffered read 4096 from 0 smp_wmb()
ext4_file_read_iter set_bit(PG_uptodate, folio_flags)
generic_file_read_iter i_size_write // 2048
filemap_read unlock_page(page)
filemap_get_pages
filemap_get_read_batch
folio_test_uptodate(folio)
ret = test_bit(PG_uptodate, folio_flags)
if (ret)
smp_rmb();
// Ensure that the data in page 0-2048 is up-to-date.
// New buffered write 2048 from 2048
ext4_buffered_write_iter
generic_perform_write
copy_page_from_iter_atomic
ext4_da_write_end
ext4_da_do_write_end
block_write_end
__block_commit_write
folio_mark_uptodate
smp_wmb()
set_bit(PG_uptodate, folio_flags)
i_size_write // 4096
unlock_page(page)
isize = i_size_read(inode) // 4096
// Read the latest isize 4096, but without smp_rmb(), there may be
// Load-Load disorder resulting in the data in the 2048-4096 range
// in the page is not up-to-date.
copy_page_to_iter
// copyout 4096
In the concurrency above, we read the updated i_size, but there is no read
barrier to ensure that the data in the page is the same as the i_size at
this point, so we may copy the unsynchronized page out. Hence adding the
missing read memory barrier to fix this.
This is a Load-Load reordering issue, which only occurs on some weak
mem-ordering architectures (e.g. ARM64, ALPHA), but not on strong
mem-ordering architectures (e.g. X86). And theoretically the problem
doesn't only happen on ext4, filesystems that call filemap_read() but
don't hold inode lock (e.g. btrfs, f2fs, ubifs ...) will have this
problem, while filesystems with inode lock (e.g. xfs, nfs) won't have
this problem.
Cc: stable@xxxxxxxxxx
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
V1->V2:
Change the comment to the one suggested by Jan Kara.
mm/filemap.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/mm/filemap.c b/mm/filemap.c
index 71f00539ac00..10c4583c06ce 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2607,6 +2607,15 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
goto put_folios;
end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
+ /*
+ * Pairs with a barrier in
+ * block_write_end()->mark_buffer_dirty() or other page
+ * dirtying routines like iomap_write_end() to ensure
+ * changes to page contents are visible before we see
+ * increased inode size.
+ */
+ smp_rmb();
+
/*
* Once we start copying data, we don't want to be touching any
* cachelines that might be contended:
--
2.31.1
