blocks|key|75009|text|注意:我的答案与内核版本<+3.14有关，因为3.14API略有变化。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|75010|在内核中，您可以使用struct+bio读写某些扇区。此结构用于所有块级I/O。综合文档可以在内核中和奥恩上找到。这些是这一结构中最重要的几个成员：|offset|length|style|CODE|75011|bio->bi_sector+-块I/O请求的第一扇区|unordered-list-item|75012|bio->bi_size+-+I/O请求的大小|75013|bio->bi_bdev+-读写设备|75014|bio->bi_end_io+-内核将在请求结束时调用的回调|75015|在设备映射器目标中所做的工作是映射传入的创建设备映射器目标时，至少提供两个回调：ctr和map。例如，最简单的设备映射器目标dm声明它是回调如下所示|BOLD|75016|static+struct+target_type+zero_target+=+{
+++++++++.name+++=+"zero",
+++++++++.version+=+{1,+1,+0},
+++++++++.module+=+THIS_MODULE,
+++++++++.ctr++++=+zero_ctr,
+++++++++.map++++=+zero_map,
};|code-block|syntax|javascript|75017|map是一个关键的回调--它是每个设备映射器目标的心脏。map接收传入的bio，它可以使用它做任何事情。例如，dm线性只通过预先定义的偏移量移动每个传入bio的移位扇区.见守则：|75018|static+sector_t+linear_map_sector(struct+dm_target+*ti,+sector_t+bi_sector)
{
++++++++struct+linear_c+*lc+=+ti->private;

++++++++return+lc->start+%2B+dm_target_offset(ti,+bi_sector);
}

static+void+linear_map_bio(struct+dm_target+*ti,+struct+bio+*bio)
{
++++++++struct+linear_c+*lc+=+ti->private;

++++++++bio->bi_bdev+=+lc->dev->bdev;
++++++++if+(bio_sectors(bio))
++++++++++++++++bio->bi_sector+=+linear_map_sector(ti,+bio->bi_sector);
}

static+int+linear_map(struct+dm_target+*ti,+struct+bio+*bio)
{
++++++++linear_map_bio(ti,+bio);

++++++++return+DM_MAPIO_REMAPPED;
}|75019|因为map接收到指向bio的指针，所以它可以在该指针下更改值，仅此而已。|75020|这就是映射I/O请求的方式。如果您想要创建自己的请求，那么您必须分配bio，填充它的扇区、设备、大小、结束回调和添加缓冲区来读取/写入。基本上，这只是几个步骤：|75021|调用bio_alloc分配bio。|75022|集bio->bi_bdev，bio->bi_sector，bio->bi_size，bio->bi_end_io于一身|75023|通过bio_add_page添加页面。|75024|打给submit_bio。|75025|处理bio->bi_end_io回调中的结果和错误|75026|例子可以在crypt_alloc_buffer函数中的dm-crypt目标中找到.|75027|entityMap|0|LINK|mutability|MUTABLE|url|http://lxr.free-electrons.com/source/include/linux/blk_types.h?v=3.13#L35|1|http://lxr.free-electrons.com/source/Documentation/block/biodoc.txt?v=3.13|2|http://lwn.net/Articles/26404/|3|http://lxr.free-electrons.com/source/drivers/md/dm-zero.c?v=3.13#L56|4|http://lxr.free-electrons.com/source/drivers/md/dm-crypt.c?v=3.13#L983^0|0|A|A|A|A|0|1B|3|1|1F|2|2|0|0|E|0|0|C|0|0|C|0|0|E|0|F|5|14|3|18|3|1Y|4|3|0|0|0|3|S|3|10|3|24|3|0|0|A|3|0|Y|3|0|0|1|C|E|E|T|C|16|E|0|2|C|0|2|A|0|2|E|0|5|I|5|I|4|0^^$0|@$1|2|3|4|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1Z|8|@$D|20|E|21|F|G]]|9|@$D|22|E|23|1|24]|$D|25|E|26|1|27]|$D|28|E|29|1|2A]]|A|$]]|$1|H|3|I|5|J|7|2B|8|@$D|2C|E|2D|F|G]]|9|@]|A|$]]|$1|K|3|L|5|J|7|2E|8|@$D|2F|E|2G|F|G]]|9|@]|A|$]]|$1|M|3|N|5|J|7|2H|8|@$D|2I|E|2J|F|G]]|9|@]|A|$]]|$1|O|3|P|5|J|7|2K|8|@$D|2L|E|2M|F|G]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|2N|8|@$D|2O|E|2P|F|S]|$D|2Q|E|2R|F|G]|$D|2S|E|2T|F|G]]|9|@$D|2U|E|2V|1|2W]]|A|$]]|$1|T|3|U|5|V|7|2X|8|@]|9|@]|A|$W|X]]|$1|Y|3|Z|5|6|7|2Y|8|@$D|2Z|E|30|F|G]|$D|31|E|32|F|G]|$D|33|E|34|F|G]|$D|35|E|36|F|G]]|9|@]|A|$]]|$1|10|3|11|5|V|7|37|8|@]|9|@]|A|$W|X]]|$1|12|3|13|5|6|7|38|8|@$D|39|E|3A|F|G]]|9|@]|A|$]]|$1|14|3|15|5|6|7|3B|8|@$D|3C|E|3D|F|G]]|9|@]|A|$]]|$1|16|3|17|5|J|7|3E|8|@]|9|@]|A|$]]|$1|18|3|19|5|J|7|3F|8|@$D|3G|E|3H|F|G]|$D|3I|E|3J|F|G]|$D|3K|E|3L|F|G]|$D|3M|E|3N|F|G]]|9|@]|A|$]]|$1|1A|3|1B|5|J|7|3O|8|@$D|3P|E|3Q|F|G]]|9|@]|A|$]]|$1|1C|3|1D|5|J|7|3R|8|@$D|3S|E|3T|F|G]]|9|@]|A|$]]|$1|1E|3|1F|5|J|7|3U|8|@$D|3V|E|3W|F|G]]|9|@]|A|$]]|$1|1G|3|1H|5|6|7|3X|8|@$D|3Y|E|3Z|F|G]]|9|@$D|40|E|41|1|42]]|A|$]]|$1|1I|3|-4|5|6|7|43|8|@]|9|@]|A|$]]]|1J|$1K|$5|1L|1M|1N|A|$1O|1P]]|1Q|$5|1L|1M|1N|A|$1O|1R]]|1S|$5|1L|1M|1N|A|$1O|1T]]|1U|$5|1L|1M|1N|A|$1O|1V]]|1W|$5|1L|1M|1N|A|$1O|1X]]]]

NOTE: My answer is related to kernel version &lt; 3.14, because since 3.14 API is slightly changed.

In kernel you read/write certain sectors with <a href="http://lxr.free-electrons.com/source/include/linux/blk_types.h?v=3.13#L35" rel="noreferrer"><code>struct bio</code></a>. This struct is used for all block level I/O. Comprehensive documentation can be found <a href="http://lxr.free-electrons.com/source/Documentation/block/biodoc.txt?v=3.13" rel="noreferrer">in kernel</a> and on <a href="http://lwn.net/Articles/26404/" rel="noreferrer">lwn</a>. These are the several most significant members of this structure:

<ul>
<li><code>bio-&gt;bi_sector</code> - first sector of block I/O request</li>
<li><code>bio-&gt;bi_size</code> - size of I/O request</li>
<li><code>bio-&gt;bi_bdev</code> - device to read/write</li>
<li><code>bio-&gt;bi_end_io</code> - callback that kernel will call on the end of request</li>
</ul>

What you do in device mapper target is map incoming <code>bio</code>. When you creating your device mapper target you supply at least 2 callbacks: <code>ctr</code>, and <code>map</code>. For example, the simplest device-mapper target dm-zero declares it's callbacks <a href="http://lxr.free-electrons.com/source/drivers/md/dm-zero.c?v=3.13#L56" rel="noreferrer">as following</a>:

<pre><code>static struct target_type zero_target = {
 .name = "zero",
 .version = {1, 1, 0},
 .module = THIS_MODULE,
 .ctr = zero_ctr,
 .map = zero_map,
};
</code></pre>

<code>map</code> is a key callback - it's a heart of every device-mapper target. <code>map</code> receive incoming <code>bio</code> and it can do anything with it. For example, dm-linear just shift sector of every incoming <code>bio</code> by predefined offset. See the code:

<pre><code>static sector_t linear_map_sector(struct dm_target *ti, sector_t bi_sector)
{
 struct linear_c *lc = ti-&gt;private;

 return lc-&gt;start + dm_target_offset(ti, bi_sector);
}

static void linear_map_bio(struct dm_target *ti, struct bio *bio)
{
 struct linear_c *lc = ti-&gt;private;

 bio-&gt;bi_bdev = lc-&gt;dev-&gt;bdev;
 if (bio_sectors(bio))
 bio-&gt;bi_sector = linear_map_sector(ti, bio-&gt;bi_sector);
}

static int linear_map(struct dm_target *ti, struct bio *bio)
{
 linear_map_bio(ti, bio);

 return DM_MAPIO_REMAPPED;
}
</code></pre>

Because map receives pointer to <code>bio</code> it can change value under that pointer and that's it.

That's how you map I/O requests. If you want to create your own requests then you must allocate <code>bio</code>, fill it's sector, device, size, end callback and add buffers to read into/write from. Basically, it's just a few steps:

<ul>
<li>Call to bio_alloc to allocate bio.</li>
<li>Set <code>bio-&gt;bi_bdev</code>, <code>bio-&gt;bi_sector</code>, <code>bio-&gt;bi_size</code>, <code>bio-&gt;bi_end_io</code></li>
<li>Add pages via <code>bio_add_page</code>.</li>
<li>Call <code>submit_bio</code>.</li>
<li>Handle results and errors in <code>bio-&gt;bi_end_io</code> callback</li>
</ul>

Example can be found in dm-crypt target in <a href="http://lxr.free-electrons.com/source/drivers/md/dm-crypt.c?v=3.13#L983" rel="noreferrer"><code>crypt_alloc_buffer</code></a> function.

I am trying to implement device mapper target by referring to the already existing ones dm-linear, dm-snapshot, dm-cache etc. In my implementation, I need to perform a read/modify/write operation on a certain sector range. Since the device mapper directly talks to the block layer, I am not sure what data structures/functions to use to read the sectors in the memory, modify the buffer and write it back to another sector range.
At the application level, we have syscalls and below we have vfs_read/vfs_write. Is there anything similar for device mapper layer?
I have been stuck here for very long. Any help will be appreciated.

create device mapper target

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我试图通过引用现有的dm-线性、dm-快照、dm-cache等实现设备映射器目标。在我的实现中，我需要在某个扇区范围内执行读/修改/写操作。由于设备映射器直接与块层对话，我不确定要使用什么数据结构/函数来读取内存中的扇区，修改缓冲区并将其写回另一个扇区范围。在应用程序级别，我们有syscalls，下面是vfs_read...

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