Re: [PATCH v15 3/7] mm: Add function __putback_isolated_page

[Alexander Duyck]

On Tue, 2019-12-17 at 19:46 +0100, David Hildenbrand wrote:
> > Am 17.12.2019 um 19:25 schrieb Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>:
> > 
> > ïOn Tue, 2019-12-17 at 18:24 +0100, David Hildenbrand wrote:
> > > > > > Also there are some scenarios where __page_to_pfn is not that simple a
> > > > > > call with us having to get the node ID so we can find the pgdat structure
> > > > > > to perform the calculation. I'm not sure the compiler would be ble to
> > > > > > figure out that the result is the same for both calls, so it is better to
> > > > > > make it explicit.
> > > > > 
> > > > > Only in case of CONFIG_SPARSEMEM we have to go via the section - but I
> > > > > doubt this is really worth optimizing here.
> > > > > 
> > > > > But yeah, I'm fine with this change, only "IMHO
> > > > > get_pageblock_migratetype() would be nicer" :)
> > > > 
> > > > Aren't most distros running with CONFIG_SPARSEMEM enabled? If that is the
> > > > case why not optimize for it?
> > > 
> > > Because I tend to dislike micro-optimizations without performance
> > > numbers for code that is not on a hot path. But I mean in this case, as
> > > you said, you need the pfn either way, so it's completely fine with.
> > > 
> > > I do wonder, however, if you should just pass in the migratetype from
> > > the caller. That would be even faster ;)
> > 
> > The problem is page isolation. We can end up with a page being moved to an
> > isolate pageblock while we aren't holding the zone lock, and as such we
> > likely need to test it again anyway. So there isn't value in storing and
> > reusing the value for cases like page reporting.
> > 
> > In addition, the act of isolating the page can cause the migratetype to
> > change as __isolate_free_page will attempt to change the migratetype to
> > movable if it is one of the standard percpu types and we are pulling at
> > least half a pageblock out. So storing the value before we isolate it
> > would be problematic as well.
> > 
> > Undoing page isolation is the exception to the issues pointed out above,
> > but in that case we are overwriting the pageblock migratetype anyway so
> > the cache lines involved should all be warm from having just set the
> > value.
> 
> Nothing would speak against querying the migratetype in the caller and
> passing it on. After all youâre holding the zone lock, so it canât
> change.

That's a fair argument. I will go ahead and make that change since it only
really adds one line to patch 4 and allows us to drop several lines from
patch 3.