Re: [PATCH v3] ovl: Check link ability between upperdir and workdir

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Dec 19, 2017 at 09:49:28AM +0800, Chengguang Xu wrote:
> > 
> > 在 2017年12月19日，上午12:02，Vivek Goyal <vgoyal@redhat.com> 写道：
> > 
> > On Mon, Dec 18, 2017 at 08:55:48PM +0800, Chengguang Xu wrote:
> >> Inspired by encountering unexpected write error when
> >> upperdir and workdir having different project ids.
> > 
> > Can you please make this problem descrition little better. By
> > these two lines I really don't understand what's the problem
> > you are trying to solve. All I understood was that upperdir
> > and workdir had different project id. But not sure what
> > problem it led to and why.
> > 
> > In fact your first patch changelong was little better. It
> > atleast said that you encounter "-EXDEV". So is it rename
> > which fails. So rename is not allowed between two directories
> > having different project ids?
> > 
> 
> The background is
> I encountered an unexpected write error with error code -EXDEV in
> my environment which didn’t break any rules in overlayfs kernel
> document.
> 
> So I did some investigations and found when upperdir and workdir
> having different project quotas then rename/link operations between
> those directories would be fail with -EXDEV because project quota
> asks files in it strictly inherit project id with it’s own. This will
> make write fail during copy-up process.
> 
> I wrote first patch to check this condition, but that check seems not
> sufficient to detect write error, so after discussion with Amir,
> we decided to check link ability between upperdir and workdir, it is
> what copy-up actually doing when modifying files in lowerdirs, and also
> I decided to only print a warning message instead of directly mounting
> on read-only mode in case there is a special use case just read and do not 
> modify anything in lowerdirs.

Ok, thanks for the explanation. I really wish that some of it makes to
changelog so that somebody reading it later finds it much easier to
understand.

BTW, just curious, when upper supports O_TMPFILE, do we have to create
tmpfile in workdir/. Can we create it in upper/ and then link in
appropriate destination directory.

Amir, is there a fundamental requirement about why tmpfile creation has
to be in workdir.

Vivek