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

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

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.

> For blkdev_issue_flush() it may not be very interesting, since there's
> not much we can do about that. Just seems like very bad style to NOT
> return an error in such a case. You can assume that ordering is fine,
> but it definitely wont be in all case (eg devices that have write back
> caching on by default and don't support flush).


The thing is, you can't _do_ anything about it. So what's the point in
returning an error? The caller cannot possibly care - because there is
nothing the caller can really do.
Sure, the device may or may not re-order things, but since the caller
can't know, and can't really do a thing about it _anyway_, you're just
better off not even confusing anybody.


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