Re: [patch] improve streaming I/O [bug in shrink_mmap()]

[Rik van Riel <riel@conectiva.com.br>]

On Mon, 12 Jun 2000, Stephen C. Tweedie wrote:
> On Mon, Jun 12, 2000 at 11:46:09PM +0200, Zlatko Calusic wrote:
> > 
> > This simple one-liner solves a long standing problem in Linux VM.
> > While searching for a discardable page in shrink_mmap() Linux was too
> > easily failing and subsequently falling back to swapping. The problem
> > was that shrink_mmap() counted pages from the wrong zone, and in case
> > of balancing a relatively smaller zone (e.g. DMA zone on a 128MB
> > computer) "count" would be mistakenly spent dealing with pages from
> > the wrong zone. The net effect of all this was spurious swapping that
> > hurt performance greatly.
> 
> Nice --- it might also explain some of the excessive kswap CPU 
> utilisation we've seen reported now and again.

Indeed. And to be honest, the patch can be made even simpler.

We can simply move the test up to above the count--, so we won't
start IO for the "wrong" zones either.

There's only one serious bug left with the current shrink_mmap,
a bug which appears to be easy to trigger with this patch, but
still there without it.

Consider the case where only one zone has free_pages < pages_high,
but all the pages in the LRU queue are from the other zone or not
freeable (ie. with pagetable mapping)...

In those cases shrink_mmap() can loop forever. We probably want to
add a "maxscan" variable, initialised to nr_lru_pages, which is
decremented on every iteration through the loop to prevent us from
triggering this bug.

regards,

Rik
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