Hi Johannes,
On 03/04/2013 11:09 PM, Johannes Weiner wrote:
On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
When a page cache is to reclaim, we should to decide whether the page
cache is free.
IMO, the condition whether a page cache is free should be 3 in page
frame reclaiming. The reason lists as below.
When page is allocated, the page->_count is 1(code fragment is code-1 ).
And when the page is allocated for reading files from extern disk, the
page->_count will increment 1 by page_cache_get() in
add_to_page_cache_locked()(code fragment is code-2). When the page is to
reclaim, the isolated LRU list also increase the page->_count(code
fragment is code-3).
The page count is initialized to 1, but that does not stay with the
object. It's a reference that is passed to the allocating task, which
drops it again when it's done with the page. I.e. the pattern is like
this:
instantiation:
page = page_cache_alloc() /* instantiator reference -> 1 */
add_to_page_cache(page, mapping, offset)
get_page(page) /* page cache reference -> 2 */
lru_cache_add(page)
get_page(page) /* pagevec reference -> 3 */
/* ...initiate read, write, associate buffers, ... */
page_cache_release(page) /* drop instantiator reference -> 2 + private */
reclaim:
lru_add_drain()
page_cache_release(page) /* drop pagevec reference -> 1 + private */
IIUC, when add page to lru will lead to add to pagevec firstly, and
pagevec will take one reference, so if lru will take over the reference
taken by pagevec when page transmit from pagevec to lru? or just drop
the reference and lru will not take reference for page?
__isolate_lru_page(page)
page_cache_get(page) /* reclaim reference -> 2 + private */
is_page_cache_freeable(page)
try_to_free_buffers() /* drop buffer ref -> 2 */
__remove_mapping() /* drop page cache and isolator ref -> 0 */
free_hot_cold_page()
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