162/**
163 * mpage_readpages - populate an address space with some pages, and
164 * start reads against them.
165 *
166 * @mapping: the address_space
167 * @pages: The address of a list_head which contains the target pages. These
168 * pages have their ->index populated and are otherwise uninitialised.
169 *
170 * The page at @pages->prev has the lowest file offset, and reads should be
171 * issued in @pages->prev to @pages->next order.
172 *
173 * @nr_pages: The number of pages at *@pages
174 * @get_block: The filesystem's block mapper function.
175 *
176 * This function walks the pages and the blocks within each page, building and
177 * emitting large BIOs.
178 *
179 * If anything unusual happens, such as:
180 *
181 * - encountering a page which has buffers
182 * - encountering a page which has a non-hole after a hole
183 * - encountering a page with non-contiguous blocks
184 *
185 * then this code just gives up and calls the buffer_head-based read function.
186 * It does handle a page which has holes at the end - that is a common case:
187 * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
188 *
189 * BH_Boundary explanation:
190 *
191 * There is a problem. The mpage read code assembles several pages, gets all
192 * their disk mappings, and then submits them all. That's fine, but obtaining
193 * the disk mappings may require I/O. Reads of indirect blocks, for example.
194 *
195 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
196 * submitted in the following order:
197 * 12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
198 * because the indirect block has to be read to get the mappings of blocks
199 * 13,14,15,16. Obviously, this impacts performance.
200 *
201 * So what we do it to allow the filesystem's get_block() function to set
202 * BH_Boundary when it maps block 11. BH_Boundary says: mapping of the block
203 * after this one will require I/O against a block which is probably close to
204 * this one. So you should push what I/O you have currently accumulated.
205 *
206 * This all causes the disk requests to be issued in the correct order.
207 */