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

Jan Kara
2023-12-12T12:42:08Z

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@kernel.org
> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> ---
> 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@suse.com>
SUSE Labs, CR