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David Chinner (DC) writes:

 DC> So that mean's we'll have 2 separate mechanisms for marking
 DC> pages as delalloc. XFS uses the BH_delay flag to indicate
 DC> that a buffer (block) attached to the page is using delalloc.
 >> 
 >> well, for blocksize=pagesize we can save 56 bytes on every page.

 DC> Sure, but it means that ext4 w/ delalloc won't work on lots of
 DC> machines....

it does not currenly. but I'm going to implement that. not sure
whether it's worth to have two different codepaths for
block size=page size and block size < page size.

 DC> FWIW, how does this mechanism deal with block size < page size?
 DC> Don't you have to track delalloc on a block basis rather than
 DC> a page basis?
 >> 
 >> I'm still thinking how better to deal with that w/o much code duplication.

 DC> Code duplication in ext4, or across all filesystems?

given what Andrew said about moving the code into VFS, it's more
about all filesystems.

thanks, Alex