On Fri, May 13, 2005 at 10:55:24AM -0700, James Washer wrote:
> In do_generic_file_read(), when the page is not found, it is created, and
.. with refcount 1
> __add_to_page_cache() is called, which will in turn call page_cache_get()
> which gives us a page->count of 1, no?
That would be 2.
> Now, we "goto reapage", which calls a_op->readpage. If this readpage
> simply returns an error, without any other actions, we drop down to the
> page_cache_release(). Finding the page->count==0, we'll proceed to call
> __free_page(), which calls __free_pages() which decrements and tests
> page->count via put_page_testzero(), returning true as page->count is now
> zero... and were off to __free_pages_ok() and our panic...
>
> What did I miss? Forgive me being dense, if I'm missing something here.
> And thanks again for you help understanding this.
Think for a minute - we allocate a refcounted object. That means
getting (the only) reference to it. That means refcount equal to 1. It's
a fairly common idiom - not just pages are handled that way.
What happens is:
* We created a page. We have a reference to it.
* We put it into cache. Now there are two ways to reach it - our reference
and global search structure.
* We do ->readpage() - we know that we have a reference, so it's not going
away
* We drop our reference. That does not affect the presence in cache - it's
not our responsibility anymore. We are done with this page; if somebody
decides to kick it out of cache, it will be their resposibility to drop the
reference created when putting into cache.
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