Re: [DATAPOINT] pre7-6 will not swap

[Linus Torvalds <torvalds@transmeta.com>]

On 9 May 2000, Quintela Carreira Juan J. wrote:
> Hi Linus, 
>    I have tested two versions of the patch (against vanilla
> pre7-6), the first was to remove the test altogether (I think this is
> from Rajagopal):

I'll make my current pre7-7 available right away, to head off the
discussion.

I found out the real reason for the problem, and it was quite a lot more
subtle than I originally thought.

The "don't page out pages from zones that don't need it" test is a good
test, but it turns out that it triggers a rather serious problem: the way
the buffer cache dirty page handling is done is by having shrink_mmap() do
a "try_to_free_buffers()" on the pages it encounters that have
"page->buffer" set.

And doing that is quite important, because without that logic the buffers
don't get written to disk in a timely manner, nor do already-written
buffers get refiled to their proper lists. So you end up being "out of
memory" - not because the machine is really out of memory, but because
those buffers have a tendency to stick around if they aren't constantly
looked after by "try_to_free_buffers()".

So the real fix ended up being to re-order the tests in shrink_mmap() a
bit, so that try_to_free_buffers() is called even for pages that are on
a good zone that doesn't need any real balancing..

[ time passes ] 

pre7-7 is there now.

		Linus

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