force_page_cache_readahead

Name:Chunk the readahead into 2 megabyte units, so that we don't pin too much* memory at once.

Proto:int force_page_cache_readahead(struct address_space *mapping, struct file *filp, unsigned long offset, unsigned long nr_to_read)

Type:int

Parameter:

Type	Parameter	Name
struct address_space *	mapping
struct file *	filp
unsigned long	offset
unsigned long	nr_to_read

225

bdi = inode_to_bdi(host)

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ra = f_ra

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If Value for the false possibility is greater at compile time(!readpage && !readpages) Then Return -EINVAL

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max_pages = max_t - return maximum of two values, using the specified type*@type: data type to use*@x: first value*@y: second value(unsignedlong, max allowed IO size , Maximum readahead window )

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nr_to_read = min - return minimum of two values of the same or compatible types*@x: first value*@y: second value(nr_to_read, max_pages)

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When nr_to_read cycle

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this_chunk = 2 * 1024 * 1024 / PAGE_SIZE

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If this_chunk > nr_to_read Then this_chunk = nr_to_read

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__do_page_cache_readahead() actually reads a chunk of disk. It allocates* the pages first, then submits them for I/O. This avoids the very bad* behaviour which would occur if page allocations are causing VM writeback.

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offset += this_chunk

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nr_to_read -= this_chunk

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Return 0

**Caller**
Name	Describe
page_cache_sync_readahead	page_cache_sync_readahead - generic file readahead@mapping: address_space which holds the pagecache and I/O vectors@ra: file_ra_state which holds the readahead state@filp: passed on to ->readpage() and ->readpages()@offset: start offset into @mapping,

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