Return page_count(page) - !!page_has_private(page) == 2 discussion

(15:34:44) vincentinsz:  287 static inline int is_page_cache_freeable(struct page *page)
288 {
289         return page_count(page) - !!page_has_private(page) == 2;
290 }

(15:35:21) vincentinsz: this function eventually returns 0 or 1, right?
(15:36:35) qunying: not understand it fully, looks strange to me
(15:38:20) qunying: as !!page_has_private(page) should return 0 or 1, and  !!page_has_private(page) == 2 should always fail, then that is the result of page_count(page)
(15:40:02) vincentinsz: I thought  it is like return 3 - 1 == 2? 1 : 0 ?
(15:40:45) qunying: ah right, forgot the '-'
(15:41:11) qunying: it is always return 0 or 1
(15:42:52) vincentinsz: not sure why number 2 is special in this case ==2
(15:43:13) vincentinsz: why not == 1, or == 3 ?
(15:44:00) qunying: that is beyond my understanding, you make dig into how page_count is working
(15:53:00) vincentinsz: what the !! is for, like !!func(a), always get the oposite of function retuning value?
(15:53:47) qunying: not, it normalize the return code to 0 or 1
(15:54:02) qunying: some func(a0 may return > 1 or <> 1 to make it 1, <> 1 to make it 1, 0 to make it 0
(15:57:25) qunying: ya
(15:57:35) vincentinsz: f**k :-) so <> 0 to make it 1
(16:58:12) qunying logged out.


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(09:31:33) vincentinsz: Hi, still to the strange !!((page)->flags & ((1 <<>flags & ((1 <<>flags to something like 00010000, assuming the 1 bit value represents the PG_private, am I right?
(09:34:46) qunying: yes
(09:35:33) vincentinsz: then !!(0000100000) make it to vaule 1, right?
(09:35:59) qunying: yes
(09:38:03) vincentinsz: someone else had this explaintion: [url=http://zh-kernel.org/pipermail/linux-kernel/2009-June/011228.html]http://zh-kernel.org/pipermail/linux-kernel/2009-June/011228.html[/url]
(09:38:32) vincentinsz: is that the same thing as you said?
(09:39:27) qunying: ya
(09:40:13) vincentinsz: is that to say that !! will always get 1?
(09:40:36) qunying: no, it says none 0 value to 1
(09:40:49) qunying: 0 will always get 0
(09:51:05) vincentinsz: ok. Oh and the page_count(page) -  !!((page)->flags & ((1 <<>flag PG_private is set, then there should be another two  bit set to 1 in (page)->flags so that this page can be freeable
(09:51:53) qunying: i see
(09:53:06) vincentinsz: the other two bit could mean a page is in user mapped address space and LRU (Least recently used) list which are most likely for page reclaim candidate

(10:03:46) vincentinsz: There are many details, I could be wrong :-), the devil is the detail
(10:04:19) qunying: ^_6
(10:48:29) vincentinsz: ok, more, page_count(page) count the reference count of page, if page flag PG_private bit flag is set, the page is  pagecache page backed by inode or swap , so the pagecache itself would have 1 reference to the page, that is at least 2 ref count. Then the page has to be referenced in LRU list so it can be freed, that is 3.
(10:51:11) qunying: hmm, that is why it minors the 1 reference from private bit reference
(10:51:16) qunying: minus

and my question in zh-kernel mailing list


http://zh-kernel.org/pipermail/linux-kernel/2009-June/011426.html


Johannes Weiner has patched this function with comments to make it clear


http://marc.info/?l=linux-mm&m=124830074212169&w=2