On 30 Mar 2022, at 19:48, Zi Yan wrote:
> On 30 Mar 2022, at 19:03, Linus Torvalds wrote:
>
>> On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <[email protected]> wrote:
>>>
>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others")
>>
>> Oh, btw - should this perhaps be backported further back than that
>> alleged "fixes" commit?
>>
>> It does look like maybe the problem potentially existed before too,
>> and was just much harder to trigger.
>>
>> That said, google doesn't find any other reports that look like
>> Steven's oops, so maybe it really never happened and backporting isn't
>> called for.
>>
>> Or possibly my google-fu is just bad.
>>
>
> There might not be any issue with the original code because this bug
> could only be triggered when CONFIG_FLATMEM and CONFIG_MEMORY_ISOLATION
> are both set, which never happens, since CONFIG_MEMORY_ISOLATION
> depends on CONFIG_SPARSEMEM.
>
> By checking Steven's boot log, it should be PFN 0x21ee00 that triggers
> the bug, since the physical memory range ends at PFN 0x21edff.
> PFN 0x21ee00 is 2MB aligned instead of MAX_ORDER-1 (4MB) aligned.
> The original code assumes all physical memory ranges are at least
> MAX_ORDER-1 aligned, which is true when CONFIG_SPARSEMEM is set
> (CONFIG_MEMORY_ISOLATION depends on it), since CONFIG_SPARSEMEM
> allocates pageblock_flags array (the NULL-deferenced bitmap points
> to) at section size granularity (128MB > 4MB). However, CONFIG_FLATMEM
> does not do this. It allocates pageblock_flags array at the exact size
> of the physical memory. So checking 0x21ee00 will not cause NULL
> dereferencing when CONFIG_MEMORY_ISOLATION is set and the original
> if statement can be true.
>
> Now I am wondering if the page_is_buddy() check is correct for
> CONFIG_FLATMEM. Is mem_map allocation aligned to MAX_ORDER-1
> or just the present physical memory range? Is PageBuddy(0x21ee00)
> accessing some random memory location?
OK. mem_map seems to be MAX_ORDER-1 aligned, so there is no
problem with PageBuddy(0x21ee00).
--
Best Regards,
Yan, Zi
On 3/31/22 02:10, Zi Yan wrote:
> On 30 Mar 2022, at 19:48, Zi Yan wrote:
>
>> On 30 Mar 2022, at 19:03, Linus Torvalds wrote:
>>
>>> On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <[email protected]> wrote:
>>>>
>>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others")
>>>
>>> Oh, btw - should this perhaps be backported further back than that
>>> alleged "fixes" commit?
>>>
>>> It does look like maybe the problem potentially existed before too,
>>> and was just much harder to trigger.
>>>
>>> That said, google doesn't find any other reports that look like
>>> Steven's oops, so maybe it really never happened and backporting isn't
>>> called for.
>>>
>>> Or possibly my google-fu is just bad.
>>>
>>
>> There might not be any issue with the original code because this bug
>> could only be triggered when CONFIG_FLATMEM and CONFIG_MEMORY_ISOLATION
>> are both set, which never happens, since CONFIG_MEMORY_ISOLATION
>> depends on CONFIG_SPARSEMEM.
Good point. Which means unset_migratetype_isolate() that Linus pointed
out, is currently also safe as it's a CONFIG_MEMORY_ISOLATION code.
We could still implement the suggested page_find_buddy() wrapper using
page_is_buddy() internally, as well as the cleanup of __free_one_page(),
but it's not urgent.
>> By checking Steven's boot log, it should be PFN 0x21ee00 that triggers
>> the bug, since the physical memory range ends at PFN 0x21edff.
>> PFN 0x21ee00 is 2MB aligned instead of MAX_ORDER-1 (4MB) aligned.
>> The original code assumes all physical memory ranges are at least
>> MAX_ORDER-1 aligned, which is true when CONFIG_SPARSEMEM is set
>> (CONFIG_MEMORY_ISOLATION depends on it), since CONFIG_SPARSEMEM
>> allocates pageblock_flags array (the NULL-deferenced bitmap points
>> to) at section size granularity (128MB > 4MB). However, CONFIG_FLATMEM
>> does not do this. It allocates pageblock_flags array at the exact size
>> of the physical memory. So checking 0x21ee00 will not cause NULL
>> dereferencing when CONFIG_MEMORY_ISOLATION is set and the original
>> if statement can be true.
>>
>> Now I am wondering if the page_is_buddy() check is correct for
>> CONFIG_FLATMEM. Is mem_map allocation aligned to MAX_ORDER-1
>> or just the present physical memory range? Is PageBuddy(0x21ee00)
>> accessing some random memory location?
>
> OK. mem_map seems to be MAX_ORDER-1 aligned, so there is no
> problem with PageBuddy(0x21ee00).
Yeah mem_map has to be in all config variants, otherwise buddy merging
would have been blowing up in page_is_buddy() even prior to all the
"sometimes avoid merging pageblock" changes.
>
> --
> Best Regards,
> Yan, Zi
On 31 Mar 2022, at 4:57, Vlastimil Babka wrote:
> On 3/31/22 02:10, Zi Yan wrote:
>> On 30 Mar 2022, at 19:48, Zi Yan wrote:
>>
>>> On 30 Mar 2022, at 19:03, Linus Torvalds wrote:
>>>
>>>> On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <[email protected]> wrote:
>>>>>
>>>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others")
>>>>
>>>> Oh, btw - should this perhaps be backported further back than that
>>>> alleged "fixes" commit?
>>>>
>>>> It does look like maybe the problem potentially existed before too,
>>>> and was just much harder to trigger.
>>>>
>>>> That said, google doesn't find any other reports that look like
>>>> Steven's oops, so maybe it really never happened and backporting isn't
>>>> called for.
>>>>
>>>> Or possibly my google-fu is just bad.
>>>>
>>>
>>> There might not be any issue with the original code because this bug
>>> could only be triggered when CONFIG_FLATMEM and CONFIG_MEMORY_ISOLATION
>>> are both set, which never happens, since CONFIG_MEMORY_ISOLATION
>>> depends on CONFIG_SPARSEMEM.
>
> Good point. Which means unset_migratetype_isolate() that Linus pointed
> out, is currently also safe as it's a CONFIG_MEMORY_ISOLATION code.
> We could still implement the suggested page_find_buddy() wrapper using
> page_is_buddy() internally, as well as the cleanup of __free_one_page(),
> but it's not urgent.
>
Sure. Will do that.
>>> By checking Steven's boot log, it should be PFN 0x21ee00 that triggers
>>> the bug, since the physical memory range ends at PFN 0x21edff.
>>> PFN 0x21ee00 is 2MB aligned instead of MAX_ORDER-1 (4MB) aligned.
>>> The original code assumes all physical memory ranges are at least
>>> MAX_ORDER-1 aligned, which is true when CONFIG_SPARSEMEM is set
>>> (CONFIG_MEMORY_ISOLATION depends on it), since CONFIG_SPARSEMEM
>>> allocates pageblock_flags array (the NULL-deferenced bitmap points
>>> to) at section size granularity (128MB > 4MB). However, CONFIG_FLATMEM
>>> does not do this. It allocates pageblock_flags array at the exact size
>>> of the physical memory. So checking 0x21ee00 will not cause NULL
>>> dereferencing when CONFIG_MEMORY_ISOLATION is set and the original
>>> if statement can be true.
For the record, the actual situation is that struct page of PFN 0x21ee00
is allocated but uninitialized. In CONFIG_FLATMEM, pageblock flags are
stored in zone->pageblock_flags. When reading PFN 0x21ee00's pageblock
flags, the page's zone is determined to be ZONE_MOVABLE, since page->flags
is -1UL and page_zonenum() is 3. The system does not have ZONE_MOVABLE,
so zone->pageblock_flags is NULL and reading it caused NULL pointer
dereferencing.
>>>
>>> Now I am wondering if the page_is_buddy() check is correct for
>>> CONFIG_FLATMEM. Is mem_map allocation aligned to MAX_ORDER-1
>>> or just the present physical memory range? Is PageBuddy(0x21ee00)
>>> accessing some random memory location?
>>
>> OK. mem_map seems to be MAX_ORDER-1 aligned, so there is no
>> problem with PageBuddy(0x21ee00).
>
> Yeah mem_map has to be in all config variants, otherwise buddy merging
> would have been blowing up in page_is_buddy() even prior to all the
> "sometimes avoid merging pageblock" changes.
>>
>> --
>> Best Regards,
>> Yan, Zi
--
Best Regards,
Yan, Zi