Re: [PATCH 2/3] f2fs: support finding extents after isize

[Fan Li <fanofcode.li@samsung.com>]

> -----Original Message-----
> From: Chao Yu [mailto:chao@kernel.org]
> Sent: Wednesday, December 30, 2015 9:28 PM
> To: Fan Li; 'Jaegeuk Kim'
> Cc: linux-f2fs-devel@lists.sourceforge.net
> Subject: Re: [f2fs-dev] [PATCH 2/3] f2fs: support finding extents after isize
> 
> Hi,
> 
> On 12/30/15 5:17 PM, Fan Li wrote:
> > f2fs allows preallocation beyond isize, but f2fs_fiemap only look up
> > extents within isize. Therefore add this support.
> >
> > Note: It's possible that there are holes after isize, for example,
> > fallocate  multiple discontinuous extents after isize with
> > FALLOC_FL_KEEP_SIZE set. Since I can tell no differences between EOF
> > and holes from return of get_data_block, I'm afaid this patch can't
> > support such scenarios.
> 
> As you mentioned, preallocated block beyond isize can be allocated in f2fs, and we are trying to support mapping extents across
> whole data space of inode, so why we treat theses extents inside i_size and outside i_size separately? IMO, instead using i_size,
we
> should use max blocks as boundary.
> 
> Most important, this interface still can't support finding all extents after i_size, which looks buggy for our user.

Notice that this issue exists before my patch, by adding this patch, at least now it can support more scenarios
such as  fallocate a range right after isize. I'd say it's an improvement.

use max blocks as boundary would get us the precise result, but it also means after we reach the EOF, we still
need to look up every block between the EOF and  sb-> s_maxbytes to make sure the EOF is true, that's 
about 4TB or 10^9 blocks.
And it affects all scenarios where the search range covers the last extent in the file, not just extents beyond
isize. I think this price is too high to pay. 

I was hoping that I can make f2fs_map_block return an EOF to solve this problem some time later, or anyone
have a better idea?

> 
> Thanks,
> 
> >
> >
> > Signed-off-by: Fan li <fanofcode.li@samsung.com>
> > ---
> >  fs/f2fs/data.c |    7 +------
> >  1 file changed, 1 insertion(+), 6 deletions(-)
> >
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c index a9a4d89..f89cf07
> > 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -798,12 +798,6 @@ int f2fs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> >  	isize = i_size_read(inode);
> >
> >  	mutex_lock(&inode->i_mutex);
> > -	if (start >= isize)
> > -		goto out;
> > -
> > -	if (start + len > isize)
> > -		len = isize - start;
> > -
> >  	if (logical_to_blk(inode, len) == 0)
> >  		len = blk_to_logical(inode, 1);
> >
> > @@ -829,6 +823,7 @@ next:
> >  		 * punch holes beyond isize and keep size unchanged.
> >  		 */
> >  		flags |= FIEMAP_EXTENT_LAST;
> > +		last_blk = start_blk - 1;
> >  	}
> >
> >  	if (size)
> >


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