Re: Note describing poor dcache utilization under high memory pressure

[Rick Stevens <rstevens@vitalstream.com>]

Rik van Riel wrote:

> On Mon, 28 Jan 2002, Rick Stevens wrote:
> 
>>Daniel Phillips wrote:
>>[snip]
>>
>   [page table COW description]
> 
> 
>>Perhaps I'm missing this, but I read that as the child gets a reference
>>to the parent's memory.  If the child attempts a write, then new memory
>>is allocated, data copied and the write occurs to this new memory.  As
>>I read this, it's only invoked on a child write.
>>
>>Would this not leave a hole where the parent could write and, since the
>>child shares that memory, the new data would be read by the child?  Sort
>>of a hidden shm segment?  If so, I think we've got problems brewing.
>>Now, if a parent write causes the same behaviour as a child write, then
>>my point is moot.
>>
> 
> Daniel and I discussed this issue when Daniel first came up with
> the idea of doing page table COW.  He seemed a bit confused by
> fork semantics when we first discussed this idea, too ;)
> 
> You're right though, both parent and child need to react in the
> same way, preferably _without_ having to walk all of the parent's
> page tables and mark them read-only ...


Ah, good!  I wasn't losing it then.  That's a relief!

I've gotta read up on the kernel's VM system.  I use to write them
for a certain three-letter-acronymed company--many, many moons ago.
Maybe I'd have some ideas.  Then again, perhaps not.

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