Messages in this thread | | | Date | Sun, 6 Feb 2022 00:09:39 +0800 | Subject | Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation | From | Chengguang Xu <> |
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在 2021/12/7 13:33, Amir Goldstein 写道: > On Sun, Dec 5, 2021 at 4:07 PM Chengguang Xu <cgxu519@mykernel.net> wrote: >> ---- 在 星期四, 2021-12-02 06:47:25 Amir Goldstein <amir73il@gmail.com> 撰写 ---- >> > On Wed, Dec 1, 2021 at 6:24 PM Chengguang Xu <cgxu519@mykernel.net> wrote: >> > > >> > > ---- 在 星期三, 2021-12-01 21:46:10 Jan Kara <jack@suse.cz> 撰写 ---- >> > > > On Wed 01-12-21 09:19:17, Amir Goldstein wrote: >> > > > > On Wed, Dec 1, 2021 at 8:31 AM Chengguang Xu <cgxu519@mykernel.net> wrote: >> > > > > > So the final solution to handle all the concerns looks like accurately >> > > > > > mark overlay inode diry on modification and re-mark dirty only for >> > > > > > mmaped file in ->write_inode(). >> > > > > > >> > > > > > Hi Miklos, Jan >> > > > > > >> > > > > > Will you agree with new proposal above? >> > > > > > >> > > > > >> > > > > Maybe you can still pull off a simpler version by remarking dirty only >> > > > > writably mmapped upper AND inode_is_open_for_write(upper)? >> > > > >> > > > Well, if inode is writeably mapped, it must be also open for write, doesn't >> > > > it? The VMA of the mapping will hold file open. So remarking overlay inode >> > > > dirty during writeback while inode_is_open_for_write(upper) looks like >> > > > reasonably easy and presumably there won't be that many inodes open for >> > > > writing for this to become big overhead? >> > >> > I think it should be ok and a good tradeoff of complexity vs. performance. >> >> IMO, mark dirtiness on write is relatively simple, so I think we can mark the >> overlayfs inode dirty during real write behavior and only remark writable mmap >> unconditionally in ->write_inode(). >> > If by "on write" you mean on write/copy_file_range/splice_write/... > then yes I agree > since we have to cover all other mnt_want_write() cases anyway. > >> > >> > > > >> > > > > If I am not mistaken, if you always mark overlay inode dirty on ovl_flush() >> > > > > of FMODE_WRITE file, there is nothing that can make upper inode dirty >> > > > > after last close (if upper is not mmaped), so one more inode sync should >> > > > > be enough. No? >> > > > >> > > > But we still need to catch other dirtying events like timestamp updates, >> > > > truncate(2) etc. to mark overlay inode dirty. Not sure how reliably that >> > > > can be done... >> > > > >> > >> > Oh yeh, we have those as well :) >> > All those cases should be covered by ovl_copyattr() that updates the >> > ovl inode ctime/mtime, so always dirty in ovl_copyattr() should be good. >> >> Currently ovl_copyattr() does not cover all the cases, so I think we still need to carefully >> check all the places of calling mnt_want_write(). >> > Careful audit is always good, but if we do not have ovl_copyattr() in > a call site > that should mark inode dirty, then it sounds like a bug, because ovl inode ctime > will not get updated. Do you know of any such cases?
Sorry for my late response, I've been very busy lately. For your question, for example, there is a case of calling ovl_want_write() in ovl_cache_get_impure() and caller does not call ovl_copyattr() so I think we should explicitly mark ovl inode dirty in that case. Is that probably a bug?
Thanks, Chengguang
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