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    <p>I'm no kernel expert, but I would assume you need to keep this memory mapped while doing the comparison? In other words, don't call <code>kunmap</code> until after the <code>memcmp</code> is complete. I would presume that calling it before will result in <code>write_buffer</code> pointing to a page which is no longer mapped.</p> <p>Taking your code in the other question, here is a rough attempt at incremental. Still needs some cleanup, I'm sure:</p> <pre><code>static int compare_data(sector_t location, struct bio * bio, struct cache_c * dmc) { struct dm_io_region where; unsigned long bits; int segno; struct bio_vec * bvec; struct page * page; unsigned char * cache_data; char * temp_data; char * write_data; int length, i; int result = 0; size_t position = 0; size_t max_size = (dmc-&gt;block_size * 512) + 1; cache_data = (unsigned char *)vmalloc(max_size); where.bdev = dmc-&gt;cache_dev-&gt;bdev; where.count = dmc-&gt;block_size; where.sector = location &lt;&lt; dmc-&gt;block_shift; printk(KERN_DEBUG "place: %llu\n", where.sector); dm_io_sync_vm(1, &amp;where, READ, cache_data, &amp;bits, dmc); bio_for_each_segment(bvec, bio, segno) { // Map the page into memory page = bio_page(bio); write_data = (char *)kmap(page); length = bvec-&gt;bv_len; // Make sure we don't go past the end if(position &gt;= max_size) break; if(position + length &gt; max_size) length = max_size - position; // Compare the data result = memcmp(write_data, cache_data + position, length); position += length; kunmap(page); // If the memory is not equal, bail out now and return the result if(result != 0) break; } cache_data[dmc-&gt;block_size * 512] = '\0'; return result; } </code></pre>
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    1. COThe following link might be helpful : http://kneuro.net/linux-mm/index.php?file=kmap.html ... If I understand it correctly, it confirms your opinion about this problem.
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    2. COYou can also write it thusly: `write_buffer = kmap(page);` This may help the clarity of *map/use/unmap*
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      1. USPeter
    3. COThis solution is problematic because according to what I understand, a bio may have many segments. I want to loop over these segments and concatenate them together into one string. If have to leave each segment mapped, I would need to keep pointers to them to unmap them later. Furthermore, would writing the data into my own buffer take care of that issue anyways?
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