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

[Christoph Hellwig <hch@lst.de>]

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.

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.