Re: [PATCH v2 03/10] kexec: separate PageHighMem() and PageHighMemZone() use case

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Mon, May 04, 2020 at 09:03:56AM -0500, Eric W. Biederman wrote:
> 
> I have added in the kexec mailling list.
> 
> Looking at the patch we are discussing it appears that the kexec code
> could be doing much better in highmem situations today but is not.

Sound great!

> 
> 
> Joonsoo Kim <js1304@gmail.com> writes:
> 
> > 2020년 5월 1일 (금) 오후 11:06, Eric W. Biederman <ebiederm@xmission.com>님이 작성:
> >>
> >> js1304@gmail.com writes:
> >>
> >> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >> >
> >> > Until now, PageHighMem() is used for two different cases. One is to check
> >> > if there is a direct mapping for this page or not. The other is to check
> >> > the zone of this page, that is, weather it is the highmem type zone or not.
> >> >
> >> > Now, we have separate functions, PageHighMem() and PageHighMemZone() for
> >> > each cases. Use appropriate one.
> >> >
> >> > Note that there are some rules to determine the proper macro.
> >> >
> >> > 1. If PageHighMem() is called for checking if the direct mapping exists
> >> > or not, use PageHighMem().
> >> > 2. If PageHighMem() is used to predict the previous gfp_flags for
> >> > this page, use PageHighMemZone(). The zone of the page is related to
> >> > the gfp_flags.
> >> > 3. If purpose of calling PageHighMem() is to count highmem page and
> >> > to interact with the system by using this count, use PageHighMemZone().
> >> > This counter is usually used to calculate the available memory for an
> >> > kernel allocation and pages on the highmem zone cannot be available
> >> > for an kernel allocation.
> >> > 4. Otherwise, use PageHighMemZone(). It's safe since it's implementation
> >> > is just copy of the previous PageHighMem() implementation and won't
> >> > be changed.
> >> >
> >> > I apply the rule #2 for this patch.
> >>
> >> Hmm.
> >>
> >> What happened to the notion of deprecating and reducing the usage of
> >> highmem?  I know that we have some embedded architectures where it is
> >> still important but this feels like it flies in the face of that.
> >
> > AFAIK, deprecating highmem requires some more time and, before then,
> > we need to support it.
> 
> But it at least makes sense to look at what we are doing with highmem
> and ask if it makes sense.
> 
> >> This part of kexec would be much more maintainable if it had a proper
> >> mm layer helper that tested to see if the page matched the passed in
> >> gfp flags.  That way the mm layer could keep changing and doing weird
> >> gyrations and this code would not care.
> >
> > Good idea! I will do it.
> >
> >>
> >> What would be really helpful is if there was a straight forward way to
> >> allocate memory whose physical address fits in the native word size.
> >>
> >>
> >> All I know for certain about this patch is that it takes a piece of code
> >> that looked like it made sense, and transfroms it into something I can
> >> not easily verify, and can not maintain.
> >
> > Although I decide to make a helper as you described above, I don't
> > understand why you think that a new code isn't maintainable. It is just
> > the same thing with different name. Could you elaborate more why do
> > you think so?
> 
> Because the current code is already wrong.  It does not handle
> the general case of what it claims to handle.  When the only distinction
> that needs to be drawn is highmem or not highmem that is likely fine.
> But now you are making it possible to draw more distinctions.  At which
> point I have no idea which distinction needs to be drawn.
> 
> 
> The code and the logic is about 20 years old.  When it was written I
> don't recally taking numa seriously and the kernel only had 3 zones
> as I recall (DMA aka the now deprecated GFP_DMA, NORMAL, and HIGH).
> 
> The code attempts to work around limitations of those old zones amd play
> nice in a highmem world by allocating memory HIGH memory and not using
> it if the memory was above 4G ( on 32bit ).
> 
> Looking the kernel now has GFP_DMA32 so on 32bit with highmem we should
> probably be using that, when allocating memory.
> 

From quick investigation, unfortunately, ZONE_DMA32 isn't available on
x86 32bit now so using GFP_DMA32 to allocate memory below 4G would not
work. Enabling ZONE_DMA32 on x86 32bit would be not simple, so, IMHO, it
would be better to leave the code as it is.

> 
> 
> Further in dealing with this memory management situation we only
> have two situations we call kimage_alloc_page.
> 
> For an indirect page which must have a valid page_address(page).
> We could probably relax that if we cared to.
> 
> For a general kexec page to store the next kernel in until we switch.
> The general pages can be in high memory.
> 
> In a highmem world all of those pages should be below 32bit.
> 
> 
> 
> Given that we fundamentally have two situations my sense is that we
> should just refactor the code so that we never have to deal with:
> 
> 
> 			/* The old page I have found cannot be a
> 			 * destination page, so return it if it's
> 			 * gfp_flags honor the ones passed in.
> 			 */
> 			if (!(gfp_mask & __GFP_HIGHMEM) &&
> 			    PageHighMem(old_page)) {
> 				kimage_free_pages(old_page);
> 				continue;
> 			}
> 
> Either we teach kimage_add_entry how to work with high memory pages
> (still 32bit accessible) or we teach kimage_alloc_page to notice it is
> an indirect page allocation and to always skip trying to reuse the page
> it found in that case.
> 
> That way the code does not need to know about forever changing mm internals.

Nice! I already have seen your patch and found that above two lines
related to HIGHMEM are removed. Thanks for your help.

Thanks.