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SubjectRe: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"
On 14 March 2018 at 14:54, Michal Hocko <mhocko@kernel.org> wrote:
> On Wed 14-03-18 14:35:12, Ard Biesheuvel wrote:
>> On 14 March 2018 at 14:13, Michal Hocko <mhocko@kernel.org> wrote:
>> > Does http://lkml.kernel.org/r/20180313224240.25295-1-neelx@redhat.com
>> > fix your issue? From the debugging info you provided it should because
>> > the patch prevents jumping backwards.
>> >
>>
>> The patch does fix the boot hang.
>>
>> But I am concerned that we are papering over a fundamental flaw in
>> memblock_next_valid_pfn().
>
> It seems that memblock_next_valid_pfn is doing the right thing here. It
> is the alignment which moves the pfn back AFAICS. I am not really
> impressed about the original patch either, to be completely honest.
> It just looks awfully tricky. I still didn't manage to wrap my head
> around the original issue though so I do not have much better ideas to
> be honest.

So first of all, memblock_next_valid_pfn() never refers to its max_pfn
argument, which is odd nut easily fixed.
Then, the whole idea of substracting one so that the pfn++ will
produce the expected value is rather hacky,

But the real problem is that rounding down pfn for the next iteration
is dodgy, because early_pfn_valid() isn't guaranteed to return true
for the rounded down value. I know it is probably fine in reality, but
dodgy as hell. The same applies to the call to early_pfn_in_nid() btw

So how about something like this (apologies on Gmail's behalf for the
whitespace damage, I can resend it as a proper patch)


---------8<-----------
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3d974cb2a1a1..b89ca999ee3b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5352,28 +5352,29 @@
* function. They do not exist on hotplugged memory.
*/
if (context != MEMMAP_EARLY)
goto not_early;

- if (!early_pfn_valid(pfn)) {
+ if (!early_pfn_valid(pfn) || !early_pfn_in_nid(pfn, nid)) {
#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
/*
* Skip to the pfn preceding the next valid one (or
* end_pfn), such that we hit a valid pfn (or end_pfn)
* on our next iteration of the loop. Note that it needs
* to be pageblock aligned even when the region itself
* is not. move_freepages_block() can shift ahead of
* the valid region but still depends on correct page
* metadata.
*/
- pfn = (memblock_next_valid_pfn(pfn, end_pfn) &
- ~(pageblock_nr_pages-1)) - 1;
-#endif
+ pfn = memblock_next_valid_pfn(pfn, end_pfn);
+ if (pfn >= end_pfn)
+ break;
+ pfn &= ~(pageblock_nr_pages - 1);
+#else
continue;
+#endif
}
- if (!early_pfn_in_nid(pfn, nid))
- continue;
if (!update_defer_init(pgdat, pfn, end_pfn, &nr_initialised))
break;

#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
/*
---------8<-----------
This ensures that we enter the remainder of the loop with a properly
aligned pfn, rather than tweaking the value of pfn so it assumes the
expected value after 'pfn++'

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