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SubjectRe: kswapd and MM overload fix
Date
On Wednesday66 0666 6 June 2001 01:16, Linus Torvalds wrote:
> On Wed, 6 Jun 2001, Andrea Arcangeli wrote:
> > Anybody running on a machine with some zone empty (<16Mbyte boxes (PDAs),
> > <1G x86 with highmem enabled kernel or an arch with an iommu like alpha)
> > probably noticed that the MM was unusable on those hardware
> > configurations (as I also incidentally mentioned a few times on l-k in
> > the last months).
> >
> > Yesterday I checked and here it is the fix (included in 2.4.5aa3).
>
> I think the real problem is that zone->pages_{min,low,high} aren't
> initialized at all for empty zones. If they were initialized (to zero, of
> course), the shortage calculations would have worked automatically.
>
> Using uninitialized variables is always bad. Your patch is certainly fine,
> but I think we should also make the init code clear the zone data for
> empty zones so that these kinds of "use uninitialized stuff" things cannot
> happen. No?
>

Lets see - that zone will have no free nor inactive pages

In page_alloc.c:254 function __alloc_pages_limit
where water_mark will be zero too...
if (z->free_pages + z->inactive_clean_pages >= water_mark) {
we will attempt a lot of interesting/unnecessary stuff.
But it should be caught by the test a few lines up...
if (!z->size)
BUG();

In page_alloc.c:331 (function __alloc_pages)
if (z->free_pages >= z->pages_low) {
page = rmqueue(z, order);
if (page)
return page;

Hmm... a lot more than first meets the eye.
Note: >= matches < in another place, removing the = will leave the mm stuck...

/RogerL

--
Roger Larsson
Skellefteå
Sweden

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