On Fri, May 13, 2005 at 06:42:50AM -0700, James Washer wrote:
> Al, relax.. as I said, I don't know much about page cache code.
>
> So, let me ask a question, if I can, with out upsetting you further.
>
> You say the analysis is, ah, incorrect.
>
> Can you help me understand what a readpage routine SHOULD do with a page
> when it finds it cannot "arrange" a successful read? Is simply returning
> an error incorrect behaviour? If so, what should the readpage do?
It is a perfectly acceptable behaviour. And it works just fine - e.g.
nfs_readpage() does that in quite a few cases.
What you are missing is the fact that page_cache_release() frees the
page only when it drops the final reference. And pages are pinned
down while they are in page cache.
If you see page_cache_release() right after ->readpage() triggering that
check, you've got out of ->readpage() with
* only one reference to page remaining
* one reference to that page acquired earlier in do_generic_file_read()
and not dropped until now.
* one reference to that page acquired back when it had been put
in page cache. Matching page_cache_release() would be done when page
is removed from page cache, but places that do it would remove the page
from cache first. Which would set ->mapping to NULL.
Conclusion: something had done an unbalanced page_cache_release(). That
happened after the moment when do_generic_file_read() had found the page
and pinned it down and before the end of ->readpage(). Most likely -
->readpage() itself or something called by it.
the page, but
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