Re: [PATCH v5 1/5] mm: page_alloc: remain memblock_next_valid_pfn() on arm and arm64

[Jia He]

On 4/2/2018 3:53 PM, Ard Biesheuvel Wrote:
> On 2 April 2018 at 09:49, Jia He <hejianet@xxxxxxxxx> wrote:
> > On 4/2/2018 2:55 PM, Ard Biesheuvel Wrote:
> > > On 2 April 2018 at 04:30, Jia He <hejianet@xxxxxxxxx> wrote:
> > > > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> > > > where possible") optimized the loop in memmap_init_zone(). But it causes
> > > > possible panic bug. So Daniel Vacek reverted it later.
> > > >
> > > > But as suggested by Daniel Vacek, it is fine to using memblock to skip
> > > > gaps and finding next valid frame with CONFIG_HAVE_ARCH_PFN_VALID.
> > > >
> > > > On arm and arm64, memblock is used by default. But generic version of
> > > > pfn_valid() is based on mem sections and memblock_next_valid_pfn() does
> > > > not always return the next valid one but skips more resulting in some
> > > > valid frames to be skipped (as if they were invalid). And that's why
> > > > kernel was eventually crashing on some !arm machines.
> > > >
> > > > And as verified by Eugeniu Rosca, arm can benifit from commit
> > > > b92df1de5d28. So remain the memblock_next_valid_pfn on arm{,64} and move
> > > > the related codes to arm64 arch directory.
> > > >
> > > > Suggested-by: Daniel Vacek <neelx@xxxxxxxxxx>
> > > > Signed-off-by: Jia He <jia.he@xxxxxxxxxxxxxxxx>
> > > Hello Jia,
> > >
> > > Apologies for chiming in late.
> > no problem, thanks for your comments  ;-)
> > > If we are going to rearchitect this, I'd rather we change the loop in
> > > memmap_init_zone() so that we skip to the next valid PFN directly
> > > rather than skipping to the last invalid PFN so that the pfn++ in the
> > hmm... Maybe this macro name makes you confused
> >
> > pfn = skip_to_last_invalid_pfn(pfn);
> >
> > how about skip_to_next_valid_pfn?
> >
> > > for () results in the next value. Can we replace the pfn++ there with
> > > a function calls that defaults to 'return pfn + 1', but does the skip
> > > for architectures that implement it?
> > I am not sure I understand your question here.
> > With this patch, on !arm arches, skip_to_last_invalid_pfn is equal to (pfn),
> > and will be increased
> > when for{} loop continue. We only *skip* to the start pfn of next valid
> > region when
> > CONFIG_HAVE_MEMBLOCK and CONFIG_HAVE_ARCH_PFN_VALID(arm/arm64 supports
> > both).
> What I am saying is that the loop in memmap_init_zone
>
> for (pfn = start_pfn; pfn < end_pfn; pfn++) { ... }
>
> should be replaced by something like
>
> for (pfn = start_pfn; pfn < end_pfn; pfn = next_valid_pfn(pfn))
After further thinking, IMO, pfn = next_valid_pfn(pfn) might have impact on

memmap_init_zone loop.

e.g.context != MEMMAP_EARLY, pfn will not be checked by early_pfn_valid, thus
It will change the memhotplug logic.

So I would choose the old implementation:
		if (!early_pfn_valid(pfn)) {
			pfn = next_valid_pfn(pfn) - 1;
			continue;
		}
Any comments? Thanks

--
Cheers,
Jia