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Re: [dm-devel] [PATCH 1/2] Add userspace device-mapper target



From: Dan Smith <danms us ibm com>
Subject: Re: [dm-devel] [PATCH 1/2] Add userspace device-mapper target
Date: Fri, 09 Feb 2007 07:54:03 -0800

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> FT> We need a pointer and length (or size) but not an offset. This
> FT> enables an user-space to pass the kernel data to write. 
> 
> Why not an offset?  If the kernel knows nothing of the format of the
> metadata, then userspace needs to be able to say "Put 512 bytes from
> this buffer at sector 23 on the disk".

Oops, I thought that we are talking about what to add to
dmu_msg_map_response structure. We need an offset for this feature,
but we already have it.


> FT> You need to set bio->bi_sector. bio_map_user() just grabs user's
> FT> pages and put them into a bio. Users of blk_rq_map_user (like
> FT> SG_IO) doesn't need bio->bi_sector.
> 
> Right, ok, I've looked through scsi_ioctl.c and cdrom.c and I have a
> much better idea of how it is currently used.

Yeah, they are better examples.


> FT> Yeah, as you said, you can just allocate buffer and use it. If
> FT> buffer is properly aligned, we can do zero-copy. But if not, the
> FT> kernel allocates pages and copies user's pages
> FT> (bio_copy_user).
> 
> Right.  I think this would be very good for both performance and ease
> of use.  Keeping track of requests in userspace to properly handle the
> endio and corresponding metadata flush can be a pain.  I don't think,
> however, that we should completely get rid of the DMU_FLAG_SYNC
> behavior, because if someone wanted to use dm-userspace block block
> debugging or something, they may want to be able to intercept both the
> request and the endio.
> 
> I'll take a stab at making this change and will post it when I have
> something working...

Great! Thanks.


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