'\" t
.\"     Title: mpage_readpages
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: The Linux VFS
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "MPAGE_READPAGES" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "The Linux VFS"
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.SH "NAME"
mpage_readpages \- populate an address space with some pages & start reads against them
.SH "SYNOPSIS"
.HP \w'int\ mpage_readpages('u
.BI "int mpage_readpages(struct\ address_space\ *\ " "mapping" ", struct\ list_head\ *\ " "pages" ", unsigned\ " "nr_pages" ", get_block_t\ " "get_block" ");"
.SH "ARGUMENTS"
.PP
\fImapping\fR
.RS 4
the address_space
.RE
.PP
\fIpages\fR
.RS 4
The address of a list_head which contains the target pages\&. These pages have their \->index populated and are otherwise uninitialised\&. The page at
\fIpages\fR\->prev has the lowest file offset, and reads should be issued in
\fIpages\fR\->prev to
\fIpages\fR\->next order\&.
.RE
.PP
\fInr_pages\fR
.RS 4
The number of pages at *\fIpages\fR
.RE
.PP
\fIget_block\fR
.RS 4
The filesystem\*(Aqs block mapper function\&.
.RE
.SH "DESCRIPTION"
.PP
This function walks the pages and the blocks within each page, building and emitting large BIOs\&.
.PP
If anything unusual happens, such as:
.PP
\- encountering a page which has buffers \- encountering a page which has a non\-hole after a hole \- encountering a page with non\-contiguous blocks
.PP
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\&.
.PP
BH_Boundary explanation:
.PP
There is a problem\&. The mpage read code assembles several pages, gets all their disk mappings, and then submits them all\&. That\*(Aqs fine, but obtaining the disk mappings may require I/O\&. Reads of indirect blocks, for example\&.
.PP
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
.PP
because the indirect block has to be read to get the mappings of blocks 13,14,15,16\&. Obviously, this impacts performance\&.
.PP
So what we do it to allow the filesystem\*(Aqs
\fBget_block\fR
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\&.
.PP
This all causes the disk requests to be issued in the correct order\&.
.SH "COPYRIGHT"
.br