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

Baokun Li
2023-12-12T13:22:02Z

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@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
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
.