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

[Chao Yu <chao2.yu@samsung.com>]

Hi all,

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> Sent: Monday, January 04, 2016 2:56 PM
> To: Fan Li
> Cc: 'Chao Yu'; linux-f2fs-devel@lists.sourceforge.net
> Subject: Re: [f2fs-dev] [PATCH 2/3] f2fs: support finding extents after isize
> 
> Hello,
> 
> On Mon, Jan 04, 2016 at 01:57:27PM +0800, Fan Li wrote:
> >
> >
> > > -----Original Message-----
> > > From: Chao Yu [mailto:chao2.yu@samsung.com]
> > > Sent: Thursday, December 31, 2015 2:34 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
> > >
> > > > -----Original Message-----
> > > > From: Fan Li [mailto:fanofcode.li@samsung.com]
> > > > Sent: Thursday, December 31, 2015 11:37 AM
> > > > To: 'Chao Yu'; 'Jaegeuk Kim'
> > > > Cc: linux-f2fs-devel@lists.sourceforge.net
> > > > Subject: Re: [f2fs-dev] [PATCH 2/3] f2fs: support finding extents
> > > > after isize
> > > >
> > > >
> > > >
> > > > > -----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.
> > >
> > > Nope, what I'm talking about is *correctness* of our ->fiemap interface, but you're trying
> to avoid it by saying "support more
> > cases,
> > > it's an improvement". That doesn't make any sense to me, since correctness issue still not
> be fixed.
> >
> > I'm not sure what you mean by avoiding, I think the comment and reply I written has already
> stated the issue and
> > limitation of this patch.
> > Now there are two suggestions:
> > 1. support one more scenario, and all old scenarios are dealt like before, but it still can't
> support discontinuous extent after
> > isize.
> > 2. support all scenarios, but sacrifice performance for lots of common scenarios by checking
> about 10^9 blocks.
> 
> IMO, we can think about #2 whether there is an efficient way.
> 
> How many cases does this incur?
> One is fallocate with keeping i_size, ana other?

AFAIK, no more.

> 
> How about adding FADVISE_OVER_ISIZE to represent inode has blocks beyond i_size?
> Then, we can set this flag in fallocate and reset it in f2fs_truncate.

Append write in such inode could destroy this convention, right?

Thanks,

> 
> Thanks,
> 
> >
> > I think we can all agree both ideas have their flaws.
> > The only divergence is that I vote the first, and you the second. I think the most important
> thing is that it works fluently in most
> > scenarios, and you think is that it works in every scenarios even it's very slow.
> >
> > I think my method is more pratical, but balance between performance and utility seems to be
> an Eternal problem.
> >
> > >
> > > >
> > > > 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.
> > >
> > > That's another performance issue.
> > >
> > > >
> > > > 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?
> > >
> > > At least we can seek valid dnode block like the way llseek use.
> > >
> > > In addition, for most cases, few of i_nid[5] in f2fs_inode will be NULL, we could skip searching
> all dnode block in such
> > non-allocated
> > > indirect node, instead of searching dnode block f2fs_map_block one by one.
> > >
> > > Thanks,
> > >
> > > >
> > > > >
> > > > > 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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> > > > Linux-f2fs-devel@lists.sourceforge.net
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