Linux Kernel Architecture

Chapter 16: Page and Buffer Cache

if (!error) {

page_cache_get(page);

SetPageLocked(page);

page->mapping = mapping;

page->index = offset;

mapping->nrpages++;

}

...

return error;

}

The index in the page cache and the pointer to the address space of the page are held in the correspond-

ing elements ofstruct page(mappingandindex). Finally, the address space page count (nrpages)is

incremented by 1 because there is now one more page in the cache.

An alternative function namedadd_to_page_cache_lruwith identical prototype is also available. This

first invokesadd_to_page_cacheto add a page to the address space-specific page cache before also

adding the page to the system’s LRU cache using thelru_cache_addfunction.

16.4.2 Finding Pages

Keeping all cached pages in a radix tree data structure is especially beneficial when the kernel needs to

decide if a given page is cached or not.find_get_pageis provided for this purpose:

mm/filemap.c

struct page * find_get_page(struct address_space *mapping, pgoff_t offset)

{

struct page *page;

page = radix_tree_lookup(&mapping->page_tree, offset);

if (page)

page_cache_get(page);

return page;

}

Life is easy for the page cache because all the hard work is done by the radix tree implementation:

radix_tree_lookupfinds the desired page at a givenoffset,andpage_cache_getincrements the page’s

reference count if one was found.

However, pages will very often belong to a file. Unfortunately, positions in a file are specified as byte

offsets, not as offsets within the page cache. How can a file offset be converted into a page cache offset?

Currently, the granularity of the page cache is a single page; that is, the leaf elements of the page cache

radix tree are single pages. Future kernels might, however, increase the granularity, so assuming a page

size granularity is not valid. Instead, the macroPAGE_CACHE_SHIFTis provided. The object size for a page

cache element can be computed by 2PAGE_CACHE_SHIFT.

Converting between byte offsets in a file and page cache offsets is then a simple matter of dividing the

index byPAGE_CACHE_SHIFT:

index = ppos >> PAGE_CACHE_SHIFT;