[Drbd-dev] Re: question about drbd

[Philipp Reisner]

Am Dienstag, 31. Oktober 2006 05:00 schrieb Jacobson:
> Dear philipp:
>     I am now studying drbd code. I can not get why, in function
> _drbd_send_page(),  the page send method can not be used in case of
> PageSlab(page) == 1? I am looking forword to your response:). Many thanks.
> one of drbd fan

If I remeber correctly, pages that are controlled by slab have only a
use count of one (Slab holds that single reference) [maybe it was zero,
because slab does not use the reference counting on pages].
When zero copy send is done, the network code simply decreases the
reference count to that page by one. 

I think it was the slab code that does not like it if someone 
modifies the reference counts of the pages with are owned by 
slab.

PS: As noted in the code, the only FS at that time, was XFS, which 
    was using such pages...

-Phil
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