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

[Jan Kara]

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.

> 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

> +		smp_rmb();
> +
>  		/*
>  		 * Once we start copying data, we don't want to be touching any
>  		 * cachelines that might be contended:
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR