Hello,

On Mon, Jan 04, 2016 at 01:57:27PM +0800, Fan Li wrote:
> 
> 
> > -----Original Message-----
> > From: Chao Yu [mailto:chao2...@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...@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:c...@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?

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.

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...@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)
> > > > >
> > >
> > >
> > > ----------------------------------------------------------------------
> > > -------- _______________________________________________
> > > Linux-f2fs-devel mailing list
> > > Linux-f2fs-devel@lists.sourceforge.net
> > > https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel

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