Re: [RFC PATCH] mm/filemap: avoid buffered read/write race to read inconsistent data

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On 2023/12/12 20:41, Jan Kara wrote:
On Tue 12-12-23 17:36:34, Baokun Li wrote:
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
AFAIK x86 can also reorder loads vs loads so the problem can in theory
happen on x86 as well.
According to what I read in the perfbook at the link below,
 Loads Reordered After Loads does not happen on x86.
pdf sheet 562 corresponds to page 550,
Table 15.5: Summary of Memory Ordering
https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook-1c.2023.06.11a.pdf
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>
---
  mm/filemap.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 71f00539ac00..6324e2ac3e74 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2607,6 +2607,9 @@ 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);
+ /* Ensure that the page cache within isize is updated. */
Barries have to be in pairs to work and it is a good practice to document
this. So here I'd have comment like:
		/*
		 * 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.
		 */

								Honza
That's a very accurate description! Thanks a lot!
I will add this comment in the next version.
+		smp_rmb();
+
  		/*
  		 * Once we start copying data, we don't want to be touching any
  		 * cachelines that might be contended:
--
2.31.1

Thanks!
--
With Best Regards,
Baokun Li
.




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