Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH] hibernate / memory hotplug: always use for_each_populated_zone() | Date | Tue, 21 Jul 2009 16:11:08 +0200 |
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On Tuesday 21 July 2009, KAMEZAWA Hiroyuki wrote: > On Tue, 21 Jul 2009 09:15:08 +0200 > Heiko Carstens <heiko.carstens@de.ibm.com> wrote: > > > On Tue, Jul 21, 2009 at 07:29:58AM +1000, Nigel Cunningham wrote: > > > Hi. > > > > > > Gerald Schaefer wrote: > > > > From: Gerald Schaefer <gerald.schaefer@de.ibm.com> > > > > > > > > Use for_each_populated_zone() instead of for_each_zone() in hibernation > > > > code. This fixes a bug on s390, where we allow both config options > > > > HIBERNATION and MEMORY_HOTPLUG, so that we also have a ZONE_MOVABLE > > > > here. We only allow hibernation if no memory hotplug operation was > > > > performed, so in fact both features can only be used exclusively, but > > > > this way we don't need 2 differently configured (distribution) kernels. > > > > > > > > If we have an unpopulated ZONE_MOVABLE, we allow hibernation but run > > > > into a BUG_ON() in memory_bm_test/set/clear_bit() because hibernation > > > > code iterates through all zones, not only the populated zones, in > > > > several places. For example, swsusp_free() does for_each_zone() and > > > > then checks for pfn_valid(), which is true even if the zone is not > > > > populated, resulting in a BUG_ON() later because the pfn cannot be > > > > found in the memory bitmap. > > > > > > I agree with your logic and patch, but doesn't this also imply that the > > > s390 implementation pfn_valid should be changed to return false for > > > those pages? > > > > For CONFIG_SPARSEMEM, which s390 uses, there is no architecture specific > > pfn_valid() implementation. > > Also it looks like the semantics of pfn_valid() aren't clear. > > At least for sparsemem it means nothing but "the memmap for the section > > this page belongs to exists". So it just means the struct page for the > > pfn exists. > > Historically, pfn_valid() just means "there is a memmap." no other meanings > in any configs/archs.
Is this documented anywhere actually?
> > We still have pfn_present() for CONFIG_SPARSEMEM. But that just means > > "some pages in the section this pfn belongs to are present." > > It just exists for sparsemem internal purpose IIUC. > > > > So it looks like checking for pfn_valid() and afterwards checking > > for PG_Reserved (?) might give what one would expect. > I think so, too. If memory is offline, PG_reserved is always set. > > In general, it's expected that "page is contiguous in MAX_ORDER range" > and no memory holes in MAX_ORDER. In most case, PG_reserved is checked > for skipping not-existing memory.
PG_reserved is also set for kernel text, at least on some architectures, and for some other areas that we want to save.
> > Looks all a bit confusing to me. > > Or maybe it's just me who is confused? :) > > > IIRC, there are no generic interface to know whether there is a physical page.
We need to know that for hibernation, though.
Well, there is a mechanism for marking making address ranges that are never to be saved, but they need to be known during initialisation already.
> pfn_valid() is only for memmap and people have used > if (pfn_valid(pfn) && !PageReserved(page)) > check. > But, hmm, If hibernation have to save PG_reserved memory, general solution is > use copy_user_page() and handle fault.
That's not exactly straightforward IMHO.
> Alternative is making use of walk_memory_resource() as memory hotplug does. > It checks resource information registered.
I'd be fine with any _simple_ mechanism allowing us to check whether there's a physical page frame for given page (or given PFN).
Best, Rafael
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