On Tue, Jan 16, 2018 at 12:06 AM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 01/15/2018 06:14 PM, Linus Torvalds wrote:
>> But I'm adding Dave Hansen explicitly to the cc, in case he has any
>> ideas. Not because I blame him, but he's touched the sparsemem code
>> fairly recently, so maybe he'd have some idea on adding sanity
>> checking to the sparsemem version of pfn_to_page().
>
> I swear I haven't touched it lately!
Heh. I did
git blame -C mm/sparse.c | grep 2017
and your name shows up at the beginning a lot because of commit
c4e1be9ec113 ("mm, sparsemem: break out of loops early").
And Michal Hocko (who shows up even more) was already on the cc.
> I'm not sure I'd go after pfn_to_page(). *Maybe* if we were close to
> the places where we've done a pfn_to_page(), but I'm not seeing those.
Fair enough. I just wanted to add debugging, looked at Tetsuo's
config, and went "no way am I adding debugging to the sparsemem case
because it's so confusing".
That said, I also started looking at "kmap_to_page()". That's
something that is *really* different with HIGHMEM, and while most of
the users are in random drivers that do crazy things, I do note that
one of the users is in mm/swap.c.
That thing goes back to commit 5a178119b0fb ("mm: add support for
direct_IO to highmem pages") and was only used for swap_writepage(),
if I read this right.
That swap_writepage() use of kmap()'ed patches was removed some time
later in commit 62a8067a7f35 ("bio_vec-backed iov_iter"), but the
crazy kmap_to_page() thing remained.
I see nothing actively wrong in there, but it really feels like a
"that is all bogus" thing to me.
> Did anyone else notice the
>
> [ 31.068198] ? vmalloc_sync_all+0x150/0x150
>
> present in a bunch of the stack traces? That should be pretty uncommon.
No, didn't notice that. And yes, vmalloc_sync_all() might be interesting.
> Is it just part of the normal do_page_fault() stack and the stack
> dumper picks up on it?
I don't think so. It should *not* happen normally. The fact that it
shows up in the trace means it happened that time.
It doesn't seem HIGHMEM-related, though. Or maybe the highmem signal
is bogus too, and it's just that Tetsuo's reproducer needs magical
timing.
Linus
