Messages in this thread | | | Date | Tue, 08 Nov 2005 17:00:27 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH]: Cleanup of __alloc_pages |
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Paul Jackson wrote: > Nick wrote: > >>The compiler will constant fold this out if it is halfway smart. > > > How could that happen - when get_page_from_freelist() is called twice, > once with skip_cpuset_chk == 0 and once with skip_cpuset_chk == 1? >
Because it is on the other side of an &&, which evaulates to a constant zero when !CONFIG_CPUSETS.
> > >>+#define ALLOC_WATERMARKS 0x01 /* check watermarks */ >>+#define ALLOC_HARDER 0x02 /* try to alloc harder */ >>+#define ALLOC_HIGH 0x04 /* __GFP_HIGH set */ >>+#define ALLOC_CPUSET 0x08 /* check for correct cpuset */ > > > Names - bless you. > > If these names were in a header, then calls to zone_watermark_ok() > from mm/vmscan.c could use them too? > > > >>+ * reclaim. Now things get more complex, so st up alloc_flags according > > > Typo: s/st/set/ > > > At first glance, I think you've expressed the cpuset flags correctly. > Well, correctly maintained their current meaning. Read on, and you > will see that I think that is not right. > > I'm just reading the raw patch, so likely I missed something here. > But it seems to me that zone_watermark_ok() is called from __alloc_pages() > only if the ALLOC_WATERMARKS flag is set, and it seems that the two > alloc_flags values ALLOC_HARDER and ALLOC_HIGH are only of use if > zone_watermark() is called. So what use is it setting ALLOC_HARDER > or ALLOC_HIGH if ALLOC_WATERMARKS is not set? If the get_page_from_freelist() > check: > if (alloc_flags & ALLOC_WATERMARKS) > was instead: > if (alloc_flags & ALLOC_WATERMARKS|ALLOC_HARDER|ALLOC_HIGH) > then this would make more sense to me. Or changing ALLOC_WATERMARKS > to ALLOC_EASY, and make it behave similarly to the HARDER & HIGH flags. > Or maybe if the initialization of alloc_flags: > >>+ alloc_flags = 0; > > was instead: > + alloc_flags = ALLOC_WATERMARKS; >
Yep that's a bug. Thanks. Maybe instead we should have a specific flag for ALLOC_NO_WATERMARKS because that is the unusual case. The use of the flag there would be a good annotation too.
> The cpuset check in the 'ignoring mins' code shortly after this for the > PF_MEMALLOC or TIF_MEMDIE cases seems bogus. This is the case where we > should be most willing to use memory, regardless of where we find it. > That cpuset check should be removed. >
OK that would be fine, but let's do that (and your suggested possible consolidation of ALLOC_CPUSET) in another patch?
> My current inclination - check cpusets in the WATERMARKS or HARDER > or (HIGH && wait) cases, but ignore cpusets in the (HIGH && !wait) or > 'ignoring mins' cases. Can "HIGH && wait" even happen ?? Are
Yes there is nothing preventing it.
> allocations either GFP_ATOMIC (aka GFP_HIGH) or (exclusive or) > GFP_WAIT, never both? Perhaps GFP_HIGH should be permanently > deleted (another cleanup) in favor of the more popular and expressive > GFP_ATOMIC, and __GFP_WAIT retired, in favor of !GFP_ATOMIC. >
Having __GFP_HIGH as its own flag gives some more flexibility. I don't think it has a downside?
> However, I appreciate your preference to separate cleanup from semantic > change. Perhaps this means leaving the ALLOC_CPUSET flag in your > cleanup patch, then one of us following on top of that with a patch to > simplify and fix the cpuset invocation semantics and a second cleanup > patch to remove ALLOC_CPUSET as a separate flag. >
That would be good. I'll send off a fresh patch with the ALLOC_WATERMARKS fixed after Rohit gets around to looking over it.
-- SUSE Labs, Novell Inc.
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