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

From: Jia He
Date: Mon Apr 02 2018 - 23:08:10 EST

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

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))

where next_valid_pfn() is simply defined as

static ulong next_valid_pfn(ulong pfn)
return pfn + 1;
Hi Ard,
Do you think a macro instead of simply fuction is better here?