On Fri, 2007-06-29 at 13:01 -0700, Andrew Morton wrote: > Guys, Mike and Sreenivasa at google are looking into implementing > fallocate() on ext2. Of course, any such implementation could and should > also be portable to ext3 and ext4 bitmapped files. > > I believe that Sreenivasa will mainly be doing the implementation work. > > > The basic plan is as follows: > > - Create (with tune2fs and mke2fs) a hidden file using one of the > reserved inode numbers. That file will be sized to have one bit for each > block in the partition. Let's call this the "unwritten block file". > > The unwritten block file will be initialised with all-zeroes > > - at fallocate()-time, allocate the blocks to the user's file (in some > yet-to-be-determined fashion) and, for each one which is uninitialised, > set its bit in the unwritten block file. The set bit means "this block > is uninitialised and needs to be zeroed out on read". > > - truncate() would need to clear out set-bits in the unwritten blocks file. > > - When the fs comes to read a block from disk, it will need to consult > the unwritten blocks file to see if that block should be zeroed by the > CPU. > > - When the unwritten-block is written to, its bit in the unwritten blocks > file gets zeroed. > > - An obvious efficiency concern: if a user file has no unwritten blocks > in it, we don't need to consult the unwritten blocks file. > > Need to work out how to do this. An obvious solution would be to have > a number-of-unwritten-blocks counter in the inode. But do we have space > for that? > > (I expect google and others would prefer that the on-disk format be > compatible with legacy ext2!) > > - One concern is the following scenario: > > - Mount fs with "new" kernel, fallocate() some blocks to a file. > > - Now, mount the fs under "old" kernel (which doesn't understand the > unwritten blocks file). > > - This kernel will be able to read uninitialised data from that > fallocated-to file, which is a security concern. > > - Now, the "old" kernel writes some data to a fallocated block. But > this kernel doesn't know that it needs to clear that block's flag in > the unwritten blocks file! > > - Now mount that fs under the "new" kernel and try to read that file. > The flag for the block is set, so this kernel will still zero out the > data on a read, thus corrupting the user's data > > So how to fix this? Perhaps with a per-inode flag indicating "this > inode has unwritten blocks". But to fix this problem, we'd require that > the "old" kernel clear out that flag. > > Can anyone propose a solution to this? > > Ah, I can! Use the compatibility flags in such a way as to prevent the > "old" kernel from mounting this filesystem at all. To mount this fs > under an "old" kernel the user will need to run some tool which will > > - read the unwritten blocks file > > - for each set-bit in the unwritten blocks file, zero out the > corresponding block > > - zero out the unwritten blocks file > > - rewrite the superblock to indicate that this fs may now be mounted > by an "old" kernel. > > Sound sane? > > - I'm assuming that there are more reserved inodes available, and that > the changes to tune2fs and mke2fs will be basically a copy-n-paste job > from the `tune2fs -j' code. Correct? > > - I haven't thought about what fsck changes would be needed. > > Presumably quite a few. For example, fsck should check that set-bits > in the unwriten blobks file do not correspond to freed blocks. If they > do, that should be fixed up. > > And fsck can check each inodes number-of-unwritten-blocks counters > against the unwritten blocks file (if we implement the per-inode > number-of-unwritten-blocks counter) > > What else should fsck do? > > - I haven't thought about the implications of porting this into ext3/4. > Probably the commit to the unwritten blocks file will need to be atomic > with the commit to the user's file's metadata, so the unwritten-blocks > file will effectively need to be in journalled-data mode. > > Or, more likely, we access the unwritten blocks file via the blockdev > pagecache (ie: use bmap, like the journal file) and then we're just > talking direct to the disk's blocks and it becomes just more fs metadata. > > - I guess resize2fs will need to be taught about the unwritten blocks > file: to shrink and grow it appropriately. > > > That's all I can think of for now - I probably missed something. > > Suggestions and thought are sought, please. > > Another approach we have been thinking is using a backing inode(per-inode-with-preallocation) to store the preallocated blocks. When user asked for preallocation on the base inode, ext2/3 create a temporary backing inode, and it's (pre)allocate the corresponding blocks in the backing inode. When writes to the base inode, and realize we need to block allocation on, before doing the fs real block allocation, it will check if the file has a backing inode stores some preallocated blocks for the same logical blocks. If so, it will transfer the preallocated blocks from backing inode to the base inode. We need to link the two inodes in some way, maybe store the backing inode number via EA in the base inode, and flag the base inode that it has a backing inode to get preallocated blocks. Since it doesn't change the block mapping on the original file until writeout, so it doesn't require a incompat feature to protect the preallocated contents to be read in "old" kernel. There some work need to be done in e2fsck to understand the backing inode. 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