Re: [RFC] [RFT] Shared /dev/zero mmaping feature

Hi Kanoj and Linus,

kanoj@google.engr.sgi.com (Kanoj Sarcar) writes:

quoted

[snip] 

I do not believe there is any good reason to expose the special
shared memory segment used as a place holder for all /dev/zero
mappings to users via ipc* commands. This special segment exists
only because we want to reduce kernel code duplication, and all the
zshmid_kernel/ zero_id checks just make sure that regular shared
memory works pretty much the way it did before. (One thing I am
unhappy about is that this special segment eats up a shm id, but
that's probably not too bad).

The appended proposal reduces code duplication and complexity a
lot. (The diff47 needs your patches against other files added.)

What you have sent is what I used as a first draft for the implementation.
The good part of it is that it reduces code duplication. The _really_ bad
part is that it penalizes users in terms of numbers of shared memory 
segments, max size of /dev/zero mappings, and limitations imposed by
shm_ctlmax/shm_ctlall/shm_ctlmni etc. I do not think taking up a 
shmid for each /dev/zero mapping is a good idea ...

Furthermore, I did not want to change behavior of information returned
by ipc* and various procfs commands, as well as swapout behavior, thus
the creation of the zmap_list. I decided a few lines of special case
checking in a handful of places was a much better option.

If the current /dev/zero stuff hampers any plans you have with shm code 
(eg page cachification), I would be willing to talk about it ...

Kanoj

I would vote to apply diff48 to the standard kernel. For me the whole
solution is still a workaround. 

Greetings
		Christoph

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