I wondered if someone could explain what is happening in
generic_file_read and the page cache.
More generically, I'd like to understand how in a file system I can
"get" a page and use it to copy date into/out of it. How do I then put
the page away? [I'm working on some experiments for a cluster file
system exploiting object based disks, see
ftp://ftp.inter-mezzo.org/pub/obd for some sample code.]
There seem to be several patterns, but I can't figure them out:
- functions modifying a page tend to lock the page, and unlock them
understandbly
- functions reading the page do a page_get -- but there is no page_put,
instead they call the un-understandable page_cache_release.
Below in the snippet from generic_page read, the normal path does:
get_page(page)
if all is well copy the data from the page
page_cache_release(page)
But the latter is in fact __free_page(page) which is defined in
page_alloc.c and is very appropriately labeled as "You really aren't
expected to understand this".
- What does that function do with the page?
- Who sets back the page count, which was increased by get_page?
- How can I operate with pages in a file system?
Surely I'm totally confused, can someone explain what I'm missing?
- Peter -
-
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