Messages in this thread | | | Subject | Re: [PATCH 00/28] Swap over NFS -v16 | From | Peter Zijlstra <> | Date | Fri, 29 Feb 2008 12:51:52 +0100 |
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On Wed, 2008-02-27 at 10:05 +0200, Pekka Enberg wrote: > Hi Peter, > > On Wed, Feb 27, 2008 at 9:58 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > 1/ I note there is no way to tell if memory returned by kmalloc is > > > from the emergency reserve - which contrasts with alloc_page > > > which does make that information available through page->reserve. > > > This seems a slightly unfortunate aspect of the interface. > > > > Yes, but alas there is no room to store such information in kmalloc(). > > That is, in a sane way. I think it was Daniel Phillips who suggested > > encoding it in the return pointer by flipping the low bit - but that is > > just too ugly and breaks all current kmalloc sites to boot. > > Why can't you add a kmem_is_emergency() to SLUB that looks up the > cache/slab/page (whatever is the smallest unit of the emergency pool > here) for the object and use that?
I made page->reserve into PG_emergency and made that bit stick for the lifetime of that page allocation. I then made kmem_is_emergency() look up the head page backing that allocation's slab and return PageEmergency().
This gives a consistent kmem_is_emergency() - that is if during the lifetime of the kmem allocation it returns true once, it must return true always.
You can then, using this properly, push the accounting into kmalloc_reserve() and kfree_reserve() (and kmem_cache_{alloc,free}_reserve).
Which yields very pretty code all round. (can make public if you like to see..)
However...
This is a stricter model than I had before, and has one ramification I'm not entirely sure I like.
It means the page remains a reserve page throughout its lifetime, which means the slab remains a reserve slab throughout its lifetime. Therefore it may never be used for !reserve allocations. Which in turn generates complexities for the partial list.
In my previous model I had the reserve accounting external to the allocation, which relaxed the strict need for consistency here, and I dropped the reserve status once we were above the page limits again.
I managed to complicate the SLUB patch with this extra constraint, by checking reserve against PageEmergency() when scanning the partial list, but gave up on SLAB.
Does this sound like something I should pursuit? I feel it might complicate the slab allocators too much..
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