Hi list, so I've had this idea stuck in my head for a while and am finally not intimidated enough my the dm API to actually give it a shot and play around. I'm just getting used to the DM internals so please apologize if I sound like an idiot, I'm just new to the DM. Something that I would like to implement first is a device mapper target that takes three block devices as input: Two equally sized devices (src1 and src2) and a separate metadata device (meta). I want to map chunks of the src devices to bits of a bitmap in the meta device. If the bit is set in the meta device decides whether src1 and src2 is returned. Sounds pretty easy and I also got surprisingly far with my little kernel module. I've so far implemented ctr, dtr, map and status. In map() I actually do the switching operation. I've looked at how dm-linear implements this and copied a lot of information. Currently I do static switching (fixed block size, ingoring the meta device). Here are my questions: - How can I read within the kernel from the block device lc->meta->bdev? If I call "read_dev_sector" from "map" this results in a deadlock, I'm guessing this is now how it's supposed to work. The bcache module must perform something similar (because it also reads and writes metadata, only much more complex), but I'll be damned but couldn't find out where the actual reading/writing is performed in the code. What are things that I should look at? - Is the ctr callback the appropriate place to fail if a logical error occurs? For example, if two src devices of dissimilar size are passed to dmsetup? - Is i_size_read(lc->src1dev->bdev->bd_inode) the correct way of determining the size of the underlying block device? If not, which function is? - Can I safely assume the logical sector size is fixed to be 512 bytes in all cases? - In the dm-linear example, bio_sectors(bio) is checked. This gives, if I understand it correctly, the size in sectors of the BIO (usually this is 8). What I don't understand is in which cases this can become zero (dm-linear has a if that checks for bio_sectors(bio) != 0). - Can I determine the size the bio in map() will have already in ctr() somehow? Can I assume it will never change if it was once determined? The reason is that for my example I need to make sure the chunk size is a integer multiple of the bio size and I would only like to check this once (in ctr) and not every time (in map). Thank you very much for helping out a complete newbie :-) Best regards, Johannes -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
