Re: upcoming kerneloops.org item: get_page_from_freelist

[David Rientjes]

On Mon, 29 Jun 2009, Mel Gorman wrote:

> page-allocator: Ensure that processes that have been OOM killed exit the page allocator
> 
> Processes that have been OOM killed set the thread flag TIF_MEMDIE. A
> process such as this is expected to exit the page allocator but in the
> event it happens to have set __GFP_NOFAIL, it potentially loops forever.
> 

That's not the expected behavior for TIF_MEMDIE, although your patch 
certainly changes that.

Your patch is simply doing

	if (test_thread_flag(TIF_MEMDIE))
		gfp_mask |= __GFP_NORETRY;

in the slowpath.

TIF_MEMDIE is supposed to allow allocations to succeed, not automatically 
fail, so that it can quickly handle its SIGKILL without getting blocked in 
the exit path seeking more memory.

> This patch checks TIF_MEMDIE when deciding whether to loop again in the
> page allocator. Such a process will now return NULL after direct reclaim
> and OOM killing have both been considered as options. The potential
> problem is that a __GFP_NOFAIL allocation can still return failure so
> callers must still handle getting returned NULL.
> 

All __GFP_NOFAIL allocations should ensure that alloc_pages() never 
returns NULL.  Although it's unfortunate, that's the requirement that 
callers have been guaranteed and until they are fixed, the page allocator 
should respect it.

I disagree with this change because it unconditionally fails allocations 
when a task has been oom killed, a scenario which should be the _highest_ 
priority for allocations to succeed since it leads to future memory 
freeing.

Additionally, this will fail all GFP_ATOMIC allocations for oom killed 
tasks if allocating without watermarks fails although pdflush may 
concurrently be doing writeback or other allocation attempts are invoking 
direct reclaim.
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