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SubjectRe: [PATCH] f2fs: Fix direct IO handling
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On 2019/11/26 17:34, Ritesh Harjani wrote:
> Hello Damien,
>
> IIUC, you are trying to fix a stale data read by DIO read for the case
> you explained in your patch w.r.t. DIO-write forced to write as buffIO.
>
> Coincidentally I was just looking at the same code path just now.
> So I do have a query to you/f2fs group. Below could be silly one, as I
> don't understand F2FS in great detail.
>
> How is the stale data by DIO read, is protected against a mmap
> writes via f2fs_vm_page_mkwrite?
>
> f2fs_vm_page_mkwrite() f2fs_direct_IO (read)
> filemap_write_and_wait_range()
> -> f2fs_get_blocks()
> -> submit_bio()
>
> -> set_page_dirty()
>
> Is above race possible with current f2fs code?
> i.e. f2fs_direct_IO could read the stale data from the blocks
> which were allocated due to mmap fault?

The faulted page is locked until the fault is fully processed so direct
IO has to wait for that to complete first.

>
> Am I missing something here?
>
> -ritesh
>
> On 11/26/19 1:27 PM, Damien Le Moal wrote:
>> f2fs_preallocate_blocks() identifies direct IOs using the IOCB_DIRECT
>> flag for a kiocb structure. However, the file system direct IO handler
>> function f2fs_direct_IO() may have decided that a direct IO has to be
>> exececuted as a buffered IO using the function f2fs_force_buffered_io().
>> This is the case for instance for volumes including zoned block device
>> and for unaligned write IOs with LFS mode enabled.
>>
>> These 2 different methods of identifying direct IOs can result in
>> inconsistencies generating stale data access for direct reads after a
>> direct IO write that is treated as a buffered write. Fix this
>> inconsistency by combining the IOCB_DIRECT flag test with the result
>> of f2fs_force_buffered_io().
>>
>> Reported-by: Javier Gonzalez <javier@javigon.com>
>> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
>> ---
>> fs/f2fs/data.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index 5755e897a5f0..8ac2d3b70022 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -1073,6 +1073,8 @@ int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *from)
>> int flag;
>> int err = 0;
>> bool direct_io = iocb->ki_flags & IOCB_DIRECT;
>> + bool do_direct_io = direct_io &&
>> + !f2fs_force_buffered_io(inode, iocb, from);
>>
>> /* convert inline data for Direct I/O*/
>> if (direct_io) {
>> @@ -1081,7 +1083,7 @@ int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *from)
>> return err;
>> }
>>
>> - if (direct_io && allow_outplace_dio(inode, iocb, from))
>> + if (do_direct_io && allow_outplace_dio(inode, iocb, from))
>> return 0;
>>
>> if (is_inode_flag_set(inode, FI_NO_PREALLOC))
>>
>
>


--
Damien Le Moal
Western Digital Research

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