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

[Jaegeuk Kim <jaegeuk@kernel.org>]

Hi,

...

> > > > > > > > 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?
> > >
> > > I'm still not very confident about this idea, so I didn't really think
> > > that through yet, inode may be a proper place.
> > > Or we just write a special function to find it, and call it only when
> > > fiemap is called and disk size isn't initiated yet. After all this isn't frequently-used.
> > >
> > > >
> > > > > 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.
> > >
> > > if flag could indicate "may", will i_disksize do a better job in the same way?
> > > let i_disksize be the end of the last extent that *may* exist beyond
> > > i_size, set it also in fallocate,when truncate, if the length is still
> > > beyond isize, we set the new length as i_disksize, so in worse scenario we won't have to search up to s_maxbytes.
> > 
> > My real concern is that there no space for i_disksize.
> 
> I see. How about combine both ideas, use flag as you propose, and if fiemap ever needs to 
> look up the last extent beyond isize, we calculate i_disksize first and cache it in f2fs_inode_info, 
> use it to determine the last extent.
> 
> This way we don't have to look up all blocks up to s_maxbytes, and need no extra space in disk.

Looks good to me.

Thanks,

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