linux-kernel - Re: [PATCH] Revert "mm/page_alloc: fix memmap_init

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Date:   Wed, 14 Mar 2018 15:54:16 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Daniel Vacek <neelx@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Paul Burton <paul.burton@...tec.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"

On 14 March 2018 at 14:54, Michal Hocko <mhocko@...nel.org> wrote:
> On Wed 14-03-18 14:35:12, Ard Biesheuvel wrote:
>> On 14 March 2018 at 14:13, Michal Hocko <mhocko@...nel.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++'