Re: [PATCH] f2fs: Fix direct IO handling

[Ritesh Harjani]

On 11/28/19 7:40 AM, Damien Le Moal wrote:
> 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.

How about below parallelism?

 f2fs_vm_page_mkwrite()		 f2fs_direct_IO (read)
 					filemap_write_and_wait_range()
	-> down_read(->i_mmap_sem);
	-> lock_page()
	-> f2fs_get_blocks()				
 					 -> submit_bio()

 	-> set_page_dirty()

Can above DIO read not expose the stale data from block which was
allocated in f2fs_vm_page_mkwrite path?


> > 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@xxxxxxxxxxx>
> > > Signed-off-by: Damien Le Moal <damien.lemoal@xxxxxxx>
> > > ---
> > >    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))