On Thu 07-10-21 20:51:47, Miklos Szeredi wrote: > On Thu, 7 Oct 2021 at 16:53, Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote: > > > > ---- 在 星期四, 2021-10-07 22:46:46 Jan Kara <jack@xxxxxxx> 撰写 ---- > > > On Thu 07-10-21 15:34:19, Miklos Szeredi wrote: > > > > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote: > > > > > > However that wasn't what I was asking about. AFAICS ->write_inode() > > > > > > won't start write back for dirty pages. Maybe I'm missing something, > > > > > > but there it looks as if nothing will actually trigger writeback for > > > > > > dirty pages in upper inode. > > > > > > > > > > > > > > > > Actually, page writeback on upper inode will be triggered by overlayfs ->writepages and > > > > > overlayfs' ->writepages will be called by vfs writeback function (i.e writeback_sb_inodes). > > > > > > > > Right. > > > > > > > > But wouldn't it be simpler to do this from ->write_inode()? > > > > > > You could but then you'd have to make sure you have I_DIRTY_SYNC always set > > > when I_DIRTY_PAGES is set on the upper inode so that your ->write_inode() > > > callback gets called. Overall I agree the logic would be probably simpler. > > > > > > > And it's not just for simplicity. The I_SYNC logic in > writeback_single_inode() is actually necessary to prevent races > between instances on a specific inode. I.e. if inode writeback is > started by background wb then syncfs needs to synchronize with that > otherwise it will miss the inode, or worse, mess things up by calling > ->write_inode() multiple times in parallel. So going throught > writeback_single_inode() is actually a must AFAICS. Yes, you are correct. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
