---- 在 星期四, 2020-10-15 20:32:24 Amir Goldstein <amir73il@xxxxxxxxx> 撰写 ---- > > > Perhaps you can combine that with the shadow overlay sbi approach. > > > Instead of dirtying overlay inode when underlying is dirtied, you can > > > "pre-dirty" overlayfs inode in higher level file ops and add them to the > > > "maybe-dirty" list (e.g. after write). > > > > > > > Main problem is we can't be notified by set_page_dirty from writable mmap. > > Meanwhile, if we dirty overlay inode then writeback will pick up dirty overlay > > inode and clear it after syncing, then overlay inode could be release at any time, > > so in the end, maybe overlay inode is released but upper inode is still dirty and > > there is no any pointer to find upper dirty inode out. > > > > But we can control whether overlay inode is release with ovl_drop_inode() > right? Can we prevent dropping overlay inode if upper inode is > inode_is_open_for_write()? > > About re-dirty, see below... > > > > > > ovl_sync_fs() can iterate the maybe-dirty list and re-dirty overlay inodes > > > if the underlying inode is still dirty on the (!wait) pass. > > > > > > As for memory mapped inodes via overlayfs (which can be dirtied without > > > notifying overlayfs) I am not sure that is a big problem in practice. > > > > > > > Yes, it's key problem here. > > > > > When an inode is writably mapped via ovarlayfs, you can flag the > > > overlay inode with "maybe-writably-mapped" and then remove > > > it from the maybe dirty list when the underlying inode is not dirty > > > AND its i_writecount is 0 (checked on write_inode() and release()). > > > > > > Actually, there is no reason to treat writably mapped inodes and > > > other dirty inodes differently - insert to suspect list on open for > > > write, remove from suspect list on last release() or write_inode() > > > when inode is no longer dirty and writable. I have to say inserting to suspect list cannot prevent dropping, thinking of the problem of previous approach that we write dirty upper inode with current->flags & PF_MEMALLOC while evicting clean overlay inode. > > > > > > Did I miss anything? > > > > > > > If we dirty overlay inode that means we have launched writeback mechanism, > > so in this case, re-dirty overlay inode in time becomes important. > > > > My idea was to use the first call to ovl_sync_fs() with 'wait' false > to iterate the > maybe-dirty list and re-dirty overlay inodes whose upper is dirty. > I'm curious how we prevent dropping of clean overlay inode with dirty upper? Hold another reference during iput_final operation? in the drop_inode() or something else? > Then in the second call to __sync_filesystem, sync_inodes_sb() will take > care of calling ovl_write_inode() for all the re-dirty inodes. > > In current code we sync ALL dirty upper fs inodes and we do not overlay > inodes with no reference in cache. > > The best code would sync only upper fs inodes dirtied by this overlay > and will be able to evict overlay inodes whose upper inodes are clean. > > The compromise code would sync only upper fs inodes dirtied by this overlay, > and would not evict overlay inodes as long as upper inodes are "open for write". > I think its a fine compromise considering the alternatives. > > Is this workable? > In your approach, the key point is how to prevent dropping overlay inode that has dirty upper and no reference but I don't understand well how to achieve it from your descriptions. Thanks, Chengguang
