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SubjectRe: [PATCH 0/7] Kill PF_MEMALLOC abuse
On Tue, 17 Nov 2009, KOSAKI Motohiro wrote:

> PF_MEMALLOC have following effects.
> (1) Ignore zone watermark
> (2) Don't call reclaim although allocation failure, instead return ENOMEM
> (3) Don't invoke OOM Killer
> (4) Don't retry internally in page alloc
> Some subsystem paid attention (1) only, and start to use PF_MEMALLOC abuse.
> But, the fact is, PF_MEMALLOC is the promise of "I have lots freeable memory.
> if I allocate few memory, I can return more much meory to the system!".
> Non MM subsystem must not use PF_MEMALLOC. Memory reclaim
> need few memory, anyone must not prevent it. Otherwise the system cause
> mysterious hang-up and/or OOM Killer invokation.
> if many subsystem will be able to use emergency memory without any
> usage rule, it isn't for emergency. it can become empty easily.
> Plus, characteristics (2)-(4) mean PF_MEMALLOC don't fit to general
> high priority memory allocation.
> Thus, We kill all PF_MEMALLOC usage in no MM subsystem.

I agree in principle with removing non-VM users of PF_MEMALLOC, but I
think it should be left to the individual subsystem maintainers to apply
or ack since the allocations may depend on the __GFP_NORETRY | ~__GFP_WAIT
behavior of PF_MEMALLOC. This could be potentially dangerous for a
PF_MEMALLOC user if allocations made by the kthread, for example, should
never retry for orders smaller than PAGE_ALLOC_COSTLY_ORDER or block on
direct reclaim.

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