From: Sven Schnelle <svens@linux.ibm.com>
To: Liam Howlett <liam.howlett@oracle.com>
Cc: Heiko Carstens <hca@linux.ibm.com>,
Guenter Roeck <linux@roeck-us.net>,
Andrew Morton <akpm@linux-foundation.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mapletree-vs-khugepaged
Date: Mon, 16 May 2022 17:37:29 +0200 [thread overview]
Message-ID: <yt9d1qwt5w06.fsf@linux.ibm.com> (raw)
In-Reply-To: <20220516140202.pcw2f6gu4kyslmjd@revolver> (Liam Howlett's message of "Mon, 16 May 2022 14:02:09 +0000")
[-- Attachment #1: Type: text/plain, Size: 2874 bytes --]
Hi Liam,
Liam Howlett <liam.howlett@oracle.com> writes:
> * Sven Schnelle <svens@linux.ibm.com> [220515 16:02]:
>
> I tried the above on my qemu s390 with kernel 5.18.0-rc6-next-20220513,
> but it runs without issue, return code is 0. Is there something the VM
> needs to have for this to trigger?
A coworker said the same. Reason for this seems to be that i've run the
code in a unittest environment which seems to make a difference. When
compiling the code above with gcc on my system it also doesn't crash.
So i have to figure out what makes this unittest binary special.
>> I've added a few debug statements to the maple tree code:
>>
>> [ 27.769641] mas_next_entry: offset=14
>> [ 27.769642] mas_next_nentry: entry = 0e00000000000000, slots=0000000090249f80, mas->offset=15 count=14
>
> Where exactly are you printing this?
I added a lot of debug statements to the code trying to understand
it. I'll attach it to this mail.
>>
>> I see in mas_next_nentry() that there's a while that iterates over the
>> (used?) slots until count is reached.`
>
> Yes, mas_next_nentry() looks for the next non-null entry in the current
> node.
>
>>After that loop mas_next_entry()
>> just picks the next (unused?) entry, which is slot 15 in that case.
>
> mas_next_entry() returns the next non-null entry. If there isn't one
> returned by mas_next_nentry(), then it will advance to the next node by
> calling mas_next_node(). There are checks in there for detecting dead
> nodes for RCU use and limit checking as well.
>
>>
>> What i noticed while scanning over include/linux/maple_tree.h is:
>>
>> struct maple_range_64 {
>> struct maple_pnode *parent;
>> unsigned long pivot[MAPLE_RANGE64_SLOTS - 1];
>> union {
>> void __rcu *slot[MAPLE_RANGE64_SLOTS];
>> struct {
>> void __rcu *pad[MAPLE_RANGE64_SLOTS - 1];
>> struct maple_metadata meta;
>> };
>> };
>> };
>>
>> and struct maple_metadata is:
>>
>> struct maple_metadata {
>> unsigned char end;
>> unsigned char gap;
>> };
>>
>> If i swap the gap and end members 0x0e00000000000000 becomes
>> 0x000e000000000000. And 0xe matches our msa->offset 14 above.
>> So it looks like mas_next() in mmap_region returns the meta
>> data for the node.
>
> If this is the case, then I think any task that has more than 14 VMAs
> would have issues. I also use mas_next_entry() in mas_find() which is
> used for the mas_for_each() macro/iterator. Can you please enable
> CONFIG_DEBUG_VM_MAPLE_TREE ? mmap.c tests the tree after pretty much
> any change and will dump useful information if there is an issue -
> including the entire tree. See validate_mm_mt() for details.
>
> You can find CONFIG_DEBUG_VM_MAPLE_TREE in the config:
> kernel hacking -> Memory debugging -> Debug VM -> Debug VM maple trees
I have both DEBUG_MAPPLE_TREE and DEBUG_VM_MAPLE_TREE enabled, but don't
see anything printed.
[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: mapple-debug.diff --]
[-- Type: text/x-diff, Size: 172 bytes --]
lib/maple_tree.c | 37 +++++++++++++++++++++++++++++++++++--
mm/mmap.c | 36 +++++++++++++++++++++++++++++-------
2 files changed, 64 insertions(+), 9 deletions(-)
next prev parent reply other threads:[~2022-05-16 15:37 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-28 17:20 [PATCH] mapletree-vs-khugepaged Guenter Roeck
2022-04-28 19:27 ` Liam Howlett
2022-04-29 12:09 ` Heiko Carstens
2022-04-29 13:01 ` Liam Howlett
2022-04-29 13:10 ` Heiko Carstens
2022-04-29 16:18 ` Liam Howlett
2022-05-02 9:10 ` Geert Uytterhoeven
2022-05-13 14:46 ` Sven Schnelle
2022-05-13 14:51 ` Sven Schnelle
2022-05-13 16:49 ` Andrew Morton
2022-05-13 17:00 ` Liam Howlett
2022-05-15 20:02 ` Sven Schnelle
2022-05-16 14:02 ` Liam Howlett
2022-05-16 15:37 ` Sven Schnelle [this message]
2022-05-16 15:50 ` Liam Howlett
2022-05-16 17:10 ` Sven Schnelle
2022-05-17 14:52 ` Liam Howlett
2022-05-17 11:53 ` Heiko Carstens
2022-05-17 12:26 ` Heiko Carstens
2022-05-17 13:23 ` Guenter Roeck
2022-05-17 15:03 ` Liam Howlett
2022-05-17 16:28 ` Guenter Roeck
2022-05-17 20:38 ` Liam Howlett
2022-05-17 14:32 ` Guenter Roeck
2022-05-19 14:35 ` Liam Howlett
2022-05-19 21:41 ` Guenter Roeck
2022-05-19 22:38 ` Liam Howlett
2022-05-30 17:38 ` Liam Howlett
2022-05-31 18:56 ` Liam Howlett
2022-06-01 19:06 ` Liam Howlett
2022-05-13 17:28 ` Guenter Roeck
2022-05-13 20:12 ` Yang Shi
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