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

[Jaegeuk Kim <jaegeuk@kernel.org>]

Hi,

On Mon, Jan 04, 2016 at 07:13:43PM +0800, Fan Li wrote:
> 
> 
> > -----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?
> > 
> > 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.
> 
> I have a similar idea that add an actual size which marks the end of last extent, so we can 
> know if the current extent is the last one, even without searching for extents behind.

Where do you want to store that size in disk?

> But there is a problem I still can't figure out,  after truncate an extent at the end of file
> beyond isize , how do I know where the new last extent ends or if there are still extents
> beyond isize? after all, the extents beyond isize could be discontinuous.

So, that's why I proposed a flag instead of a kind of i_disksize.
We can just set the flag, only if a file *may* have a extent beyond i_size
in fallocate, and unset it through f2fs_truncate.
Moreover, I don't expect that this happens so frequently.

Thanks,

> 
> > 
> > Thanks,
> > 

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