> -----Original Message----- > From: Jaegeuk Kim [mailto:jaeg...@kernel.org] > Sent: Tuesday, January 05, 2016 11:08 AM > To: Fan Li; ""@jaegeuk-2.local > Cc: 'Chao Yu'; 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? > > > > 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. > > Thanks, ------------------------------------------------------------------------------ _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
