Re: [PATCH] mm: Do not walk all of system memory during show_mem

[David Rientjes]

On Wed, 16 Oct 2013, Mel Gorman wrote:

> It has been reported on very large machines that show_mem is taking almost
> 5 minutes to display information. This is a serious problem if there is
> an OOM storm. The bulk of the cost is in show_mem doing a very expensive
> PFN walk to give us the following information
> 
> Total RAM:	Also available as totalram_pages
> Highmem pages:	Also available as totalhigh_pages
> Reserved pages:	Can be inferred from the zone structure
> Shared pages:	PFN walk required
> Unshared pages:	PFN walk required
> Quick pages:	Per-cpu walk required
> 
> Only the shared/unshared pages requires a full PFN walk but that information
> is useless. It is also inaccurate as page pins of unshared pages would
> be accounted for as shared.  Even if the information was accurate, I'm
> struggling to think how the shared/unshared information could be useful
> for debugging OOM conditions. Maybe it was useful before rmap existed when
> reclaiming shared pages was costly but it is less relevant today.
> 
> The PFN walk could be optimised a bit but why bother as the information is
> useless. This patch deletes the PFN walker and infers the total RAM, highmem
> and reserved pages count from struct zone. It omits the shared/unshared page
> usage on the grounds that it is useless.  It also corrects the reporting
> of HighMem as HighMem/MovableOnly as ZONE_MOVABLE has similar problems to
> HighMem with respect to lowmem/highmem exhaustion.
> 

We haven't been hit by this for the oom killer, but we did get hit with 
this for page allocation failure warnings as a result of having irqs 
disabled and passing GFP_ATOMIC to the page allocator without GFP_NOWARN.  
That was the intention of passing SHOW_MEM_FILTER_PAGE_COUNT into 
show_mem() in 4b59e6c47309 ("mm, show_mem: suppress page counts in 
non-blockable contexts").

With this, I assume we can just remove SHOW_MEM_FILTER_PAGE_COUNT 
entirely?
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