MPAGE_READPAGES(9)                        The Linux VFS                        MPAGE_READPAGES(9)



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

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

ARGUMENTS
       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 filesystem's block mapper function.

DESCRIPTION
       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_SIZE setups.

       BH_Boundary explanation:

       There is a problem. The mpage read code assembles several pages, gets all their disk
       mappings, and then submits them all. That's 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 submitted in
       the following order: 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 filesystem's 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.

COPYRIGHT
Kernel Hackers Manual 4.8.                 January 2017                        MPAGE_READPAGES(9)