MPAGE_READPAGES(9) The Linux VFS MPAGE_READPAGES(9)NAMEmpage_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_CACHE_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.
COPYRIGHTKernel Hackers Manual 2.6. November 2013 MPAGE_READPAGES(9)
