Re: [PATCH v2 01/34] bdev: open block device as files

Christian Brauner
2024-02-02T11:46:39Z

On Fri, Feb 02, 2024 at 07:43:24AM +0100, Christoph Hellwig wrote:
> On Thu, Feb 01, 2024 at 06:08:29PM +0100, Christian Brauner wrote:
> > > > + /*
> > > > + * O_EXCL is one of those flags that the VFS clears once it's done with
> > > > + * the operation. So don't raise it here either.
> > > > + */
> > > > + if (mode & BLK_OPEN_NDELAY)
> > > > + flags |= O_NDELAY;
> > >
> > > O_EXCL isn't dealt with in this helper at all.
> >
> > Yeah, on purpose was my point bc we can just rely on @holder and passing
> > _EXCL without holder is invalid. But I could add it.
>
> Ok. I found it weird to have the comment next to BLK_OPEN_NDELAY
> as it looked like it sneaked through. Especially as BLK_OPEN_EXCL
> has literally nothing to do with O_EXCL at all as the latter is a
> namespace operation flag. So even if the comment was intentional
> I think we're probably better off without it.
>
> > Yes, I had considered that and it would work but there's the issue that
> > we need to figure out how to handle BLK_OPEN_RESTRICT_WRITES. It has no
> > corresponding O_* flag that would let us indicate this.
>
> Oh, indeed.
>
> >
> > So I had
> > considered:
> >
> > 1/ Expose bdev_file_open_excl() so callers don't need to pass any
> > specific flags. Nearly all filesystems would effectively use this
> > helper as sb_open_mode() adds it implicitly. That would have the
> > side-effect of introducing another open helper ofc; possibly two if
> > we take _by_dev() and _by_path() into account.
> >
> > 2/ Abuse an O_* flag to mean BLK_OPEN_RESTRICT_WRITES. For example,
> > O_TRUNC or O_NOCTTY which is pretty yucky.
> >
> > 3/ Introduce an internal O_* flag which is also ugly. Vomitorious and my
> > co-maintainers would likely chop off my hands so I can't go near a
> > computer again.
> >
> > 3/ Make O_EXCL when passed together with bdev_file_open_by_*() always
> > imply BLK_OPEN_RESTRICT_WRITES.
> >
> > The 3/ option would probably be the cleanest one and I think that all
> > filesystems now pass at least a holder and holder ops so this _should_
> > work.
>
> 2 and 3 sound pretty horrible. 3 would work and look clean for the
> block side, but O_ flags are mess so I wouldn't go there.

My numbering is obviously wrong btw. That last point should obviously be 4/

>
> Maybe variant of 1 that allows for a non-exclusive open and clearly
> marks that?
>
> Or just leave it as-is for now and look into that later.

Ok.