generic_writepages & jbd2 and ext4

Hi all,

I've recently started looking into killing off the ->writepage method, and as an initial subproject kill of external uses of generic_writepages. One of the two remaining callers s in jbd2 and I'm a bit confused about it.

jbd2_journal_submit_inode_data_buffers has two comments that explicitly ask for ->writepages as that doesn't allocate data:

/*
* write the filemap data using writepage() address_space_operations.
* We don't do block allocation here even for delalloc. We don't
* use writepages() because with delayed allocation we may be doing
* block allocation in writepages().
*/

/*
* submit the inode data buffers. We use writepage
* instead of writepages. Because writepages can do
* block allocation with delalloc. We need to write
* only allocated blocks here.
*/

and these look really stange to me. ->writepage and ->writepages per their document VM/VFS semantics don't different on what they allocate, so this seems to reverse engineer ext4 internal behavior in some way. Either way looping over ->writepage just for that is rather inefficient. If jbd2 really wants a way to skip delalloc conversion can we come up with a flag in struct writeback_control for that?

Is there anyone familiar enough with this code who would be willing to give it a try?

Yes, I've written that code quite a few years ago :) And I agree JBD2 is abusing internal knowledge about ext4 here. So yes, writeback_control flag so that we can propagate the information to ->writepages method should do the trick. I'll have a look into that.

Honza

Jan Kara <[email protected]>
SUSE Labs, CR