Re: [PATCH 1/7] block: Add block_flush_device()

Chris Mason
2009-03-30T19:29:42Z

On Mon, 2009-03-30 at 15:16 -0400, Jeff Garzik wrote:
> Jens Axboe wrote:
> > On Mon, Mar 30 2009, Linus Torvalds wrote:
> >>
> >> On Mon, 30 Mar 2009, Jens Axboe wrote:
> >>> The problem is that we may not know upfront, so it sort-of has to be
> >>> this trial approach where the first barrier issued will notice and fail
> >>> with -EOPNOTSUPP.
> >> Well, absolutely. Except I don't think you shoul use ENOTSUPP, you should
> >> just set a bit in the "struct request_queue", and then return 0.
> >>
> >> IOW, something like this
> >>
> >> --- a/block/blk-barrier.c
> >> +++ b/block/blk-barrier.c
> >> @@ -318,6 +318,9 @@ int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
> >> if (!q)
> >> return -ENXIO;
> >>
> >> + if (is_queue_noflush(q))
> >> + return 0;
> >> +
> >> bio = bio_alloc(GFP_KERNEL, 0);
> >> if (!bio)
> >> return -ENOMEM;
> >> @@ -339,7 +342,7 @@ int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
> >>
> >> ret = 0;
> >> if (bio_flagged(bio, BIO_EOPNOTSUPP))
> >> - ret = -EOPNOTSUPP;
> >> + set_queue_noflush(q);
> >> else if (!bio_flagged(bio, BIO_UPTODATE))
> >> ret = -EIO;
> >>
> >>
> >> which just returns 0 if we don't support flushing on that queue.
> >>
> >> (Obviously incomplete patch, which is why I also intentionally
> >> whitespace-broke it).
> >>
> >>> Sure, we could cache this value, but it's pretty
> >>> pointless since the filesystem will stop sending barriers in this case.
> >> Well no, it won't. Or rather, it will have to have such a stupid
> >> per-filesystem flag, for no good reason.
> >
> > Sorry, I just don't see much point to doing it this way instead. So now
> > the fs will have to check a queue bit after it has issued the flush, how
> > is that any better than having the 'error' returned directly?
>
> AFAICS, the aim is simply to return zero rather than EOPNOTSUPP, for the
> not-supported case, rather than burdening all callers with such checks.
>
> Which is quite reasonable for Fernando's patch -- the direct call fsync
> case.
>
> But that leaves open the possibility that some people really do want the
> EOPNOTSUPP return value, I guess? Do existing callers need that?
>

As far as I know, reiserfs is the only one actively using it to choose
different code. It moves a single wait_on_buffer when barriers are on,
which I took out once to simplify the code. Ric saw it in some
benchmark numbers and I put it back in.

Given that it was a long time ago, I don't have a problem with changing
it to work like all the other filesystems.

-chris