Man page 9 : mpage_readpages

mpage_readpages - populate an address space with some pages & start reads against them

int mpage_readpages(struct address_space *  mapping , struct list_head *  pages , unsigned  nr_pages , get_block_t  get_block );

mapping
    the address_space

pages
    The address of a list_head which contains the target pages. These pages have their ->index populated and are otherwise uninitialised. The page at pages->prev has the lowest file offset, and reads should be issued in pages->prev to pages->next order.

nr_pages
    The number of pages at *pages

get_block
    The filesystemAqs block mapper function.

This function walks the pages and the blocks within each page, building and emitting large BIOs.

If anything unusual happens, such as:

- encountering a page which has buffers - encountering a page which has a non-hole after a hole - encountering a page with non-contiguous blocks

then this code just gives up and calls the buffer_head-based read function. It does handle a page which has holes at the end - that is a common case: the end-of-file on blocksize < PAGE_CACHE_SIZE setups.

There is a problem. The mpage read code assembles several pages, gets all their disk mappings, and then submits them all. ThatAqs fine, but obtaining the disk mappings may require I/O. Reads of indirect blocks, for example.

So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be

12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16

because the indirect block has to be read to get the mappings of blocks 13,14,15,16. Obviously, this impacts performance.

So what we do it to allow the filesystemAqs get_block function to set BH_Boundary when it maps block 11. BH_Boundary says: mapping of the block after this one will require I/O against a block which is probably close to this one. So you should push what I/O you have currently accumulated.

This all causes the disk requests to be issued in the correct order.