Linux Kernel Architecture

Chapter 8: The Virtual Filesystem

this is different from the synchronous readahead operation started before: Now the kernel does

not wait for the readahead operation to complete, but performs the reading whenever it finds

time. The readahead mechanism is considered in more detail in Chapter 16.4.5.

❑ Although the page is in the page cache, the data may not be current; this can be checked using

Page_Uptodate.

If the page is not up-to-date, it must be re-read usingmapping->a_ops->readpage. The function

pointer normally points tompage_readpage. After this call, the kernel knows for sure that the

page is filled with the most recent data.

The access to the page must be marked withmark_page_accessed; this is important to

determine page activity when it is necessary to swap data out of RAM. (Swapping is

discussed in Chapter 18.) Theactorroutine (usuallyfile_read_actor) maps the appro-

priate page into userspace address space. I won’t bother going into the details of how this

is done.^26

page still not in cache?

Usually mpage_readpage

Async

readahead

requested?

Page uptodate?

No

No

Yes

Yes

Yes

Oage not

in Page

Iterate until all desired pages have been read in

do_generic_mapping_read

find_get_page

page_cache_sync_readahead

find_get_page

page_cache_async_readahead

mapping->a_ops->readpage

no_cached_page

mark_page_accessed

actor

Figure 8-14: Code flow diagram fordo_generic_mapping_read.

If the readahead mechanism has not already read the desired page in anticipation, the function is forced
to do it itself. Theno_cached_pagesection ofdo_generic_mapping_readis used for this purpose. Its
code flow diagram is shown in Figure 8-15.^27

Oncepage_cache_alloc_coldhas reserved a cache-cold page, it is inserted in the LRU list of the
page cache viaadd_to_page_cache_lruas described in Chapter 16. Themapping->a_ops->readpage
method provided by the mapping is used to read the data. Usually, the function pointer points to
mpage_readpage, which I deal with in Chapter 16. Finally,mark_page_accessedtells the accounting
system that the page has been accessed.

(^27) Except to say that thecopy_to_userroutine discussed in Chapter 3 is used.