linux-kernel - Re: [PATCH] f2fs: Fix direct IO handling

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Message-Id: <20191126083443.F1FD5A405B@b06wcsmtp001.portsmouth.uk.ibm.com>
Date:   Tue, 26 Nov 2019 14:04:43 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Damien Le Moal <damien.lemoal@....com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, Jaegeuk Kim <jaegeuk@...nel.org>,
        Chao Yu <yuchao0@...wei.com>
Cc:     linux-fsdevel@...r.kernel.org,
        Javier Gonzalez <javier@...igon.com>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>
Subject: Re: [PATCH] f2fs: Fix direct IO handling

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?

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@...igon.com>
> Signed-off-by: Damien Le Moal <damien.lemoal@....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))
>