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SubjectRe: kernel panic in reading /proc/kpageflags when enabling RAM-simulated PMEM
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On Mon, Jun 04, 2018 at 06:18:36PM -0700, Matthew Wilcox wrote:
> On Tue, Jun 05, 2018 at 12:54:03AM +0000, Naoya Horiguchi wrote:
> > Reproduction precedure is like this:
> > - enable RAM based PMEM (with a kernel boot parameter like memmap=1G!4G)
> > - read /proc/kpageflags (or call tools/vm/page-types with no arguments)
> > (- my kernel config is attached)
> >
> > I spent a few days on this, but didn't reach any solutions.
> > So let me report this with some details below ...
> >
> > In the critial page request, stable_page_flags() is called with an argument
> > page whose ->compound_head was somehow filled with '0xffffffffffffffff'.
> > And compound_head() returns (struct page *)(head - 1), which explains the
> > address 0xfffffffffffffffe in the above message.
>
> Hm. compound_head shares with:
>
> struct list_head lru;
> struct list_head slab_list; /* uses lru */
> struct { /* Partial pages */
> struct page *next;
> unsigned long _compound_pad_1; /* compound_head */
> unsigned long _pt_pad_1; /* compound_head */
> struct dev_pagemap *pgmap;
> struct rcu_head rcu_head;
>
> None of them should be -1.
>
> > It seems that this kernel panic happens when reading kpageflags of pfn range
> > [0xbffd7, 0xc0000), which coresponds to a 'reserved' range.
> >
> > [ 0.000000] user-defined physical RAM map:
> > [ 0.000000] user: [mem 0x0000000000000000-0x000000000009fbff] usable
> > [ 0.000000] user: [mem 0x000000000009fc00-0x000000000009ffff] reserved
> > [ 0.000000] user: [mem 0x00000000000f0000-0x00000000000fffff] reserved
> > [ 0.000000] user: [mem 0x0000000000100000-0x00000000bffd6fff] usable
> > [ 0.000000] user: [mem 0x00000000bffd7000-0x00000000bfffffff] reserved
> > [ 0.000000] user: [mem 0x00000000feffc000-0x00000000feffffff] reserved
> > [ 0.000000] user: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
> > [ 0.000000] user: [mem 0x0000000100000000-0x000000013fffffff] persistent (type 12)
> >
> > So I guess 'memmap=' parameter might badly affect the memory initialization process.
> >
> > This problem doesn't reproduce on v4.17, so some pre-released patch introduces it.
> > I hope this info helps you find the solution/workaround.
>
> Can you try bisecting this? It could be one of my patches to reorder struct
> page, or it could be one of Pavel's deferred page initialisation patches.
> Or something else ;-)

Thank you for the comment. I'm trying bisecting now, let you know the result later.

And I found that my statement "not reproduce on v4.17" was wrong (I used
different kvm guests, which made some different test condition and misguided me),
this seems an older (at least < 4.15) bug.

Thanks,
Naoya Horiguchi

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