Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype

[Guo Ren]

Hi David & Zi Yan


On Thu, Jun 16, 2022 at 11:04 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 16.06.22 16:01, Zi Yan wrote:
> > On 15 Jun 2022, at 12:15, Xianting Tian wrote:
> >
> >> 在 2022/6/15 下午9:55, Zi Yan 写道:
> >>> On 15 Jun 2022, at 2:47, Xianting Tian wrote:
> >>>
> >>>> 在 2022/6/14 上午8:14, Zi Yan 写道:
> >>>>> On 13 Jun 2022, at 19:47, Guo Ren wrote:
> >>>>>
> >>>>>> On Tue, Jun 14, 2022 at 3:49 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
> >>>>>>> On 13 Jun 2022, at 12:32, Guo Ren wrote:
> >>>>>>>
> >>>>>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan <ziy@xxxxxxxxxx> wrote:
> >>>>>>>>> Hi Xianting,
> >>>>>>>>>
> >>>>>>>>> Thanks for your patch.
> >>>>>>>>>
> >>>>>>>>> On 13 Jun 2022, at 9:10, Xianting Tian wrote:
> >>>>>>>>>
> >>>>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
> >>>>>>>>>> added buddy check code. But unfortunately, this fix isn't backported to
> >>>>>>>>>> linux-5.17.y and the former stable branches. The reason is it added wrong
> >>>>>>>>>> fixes message:
> >>>>>>>>>>        Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable
> >>>>>>>>>>                            pageblocks with others")
> >>>>>>>>> No, the Fixes tag is right. The commit above does need to validate buddy.
> >>>>>>>> I think Xianting is right. The “Fixes:" tag is not accurate and the
> >>>>>>>> page_is_buddy() is necessary here.
> >>>>>>>>
> >>>>>>>> This patch could be applied to the early version of the stable tree
> >>>>>>>> (eg: Linux-5.10.y, not the master tree)
> >>>>>>> This is quite misleading. Commit 787af64d05cd applies does not mean it is
> >>>>>>> intended to fix the preexisting bug. Also it does not apply cleanly
> >>>>>>> to commit d9dddbf55667, there is a clear indentation mismatch. At best,
> >>>>>>> you can say the way of 787af64d05cd fixing 1dd214b8f21c also fixes d9dddbf55667.
> >>>>>>> There is no way you can apply 787af64d05cd to earlier trees and call it a day.
> >>>>>>>
> >>>>>>> You can mention 787af64d05cd that it fixes a bug in 1dd214b8f21c and there is
> >>>>>>> a similar bug in d9dddbf55667 that can be fixed in a similar way too. Saying
> >>>>>>> the fixes message is wrong just misleads people, making them think there is
> >>>>>>> no bug in 1dd214b8f21c. We need to be clear about this.
> >>>>>> First, d9dddbf55667 is earlier than 1dd214b8f21c in Linus tree. The
> >>>>>> origin fixes could cover the Linux-5.0.y tree if they give the
> >>>>>> accurate commit number and that is the cause we want to point out.
> >>>>> Yes, I got that d9dddbf55667 is earlier and commit 787af64d05cd fixes
> >>>>> the issue introduced by d9dddbf55667. But my point is that 787af64d05cd
> >>>>> is not intended to fix d9dddbf55667 and saying it has a wrong fixes
> >>>>> message is misleading. This is the point I want to make.
> >>>>>
> >>>>>> Second, if the patch is for d9dddbf55667 then it could cover any tree
> >>>>>> in the stable repo. Actually, we only know Linux-5.10.y has the
> >>>>>> problem.
> >>>>> But it is not and does not apply to d9dddbf55667 cleanly.
> >>>>>
> >>>>>> Maybe, Gregkh could help to direct us on how to deal with the issue:
> >>>>>> (Fixup a bug which only belongs to the former stable branch.)
> >>>>>>
> >>>>> I think you just need to send this patch without saying “commit
> >>>>> 787af64d05cd fixes message is wrong” would be a good start. You also
> >>>>> need extra fix to mm/page_isolation.c for kernels between 5.15 and 5.17
> >>>>> (inclusive). So there will need to be two patches:
> >>>>>
> >>>>> 1) your patch to stable tree prior to 5.15 and
> >>>>>
> >>>>> 2) your patch with an additional mm/page_isolation.c fix to stable tree
> >>>>> between 5.15 and 5.17.
> >>>>>
> >>>>>>> Also, you will need to fix the mm/page_isolation.c code too to make this patch
> >>>>>>> complete, unless you can show that PFN=0x1000 is never going to be encountered
> >>>>>>> in the mm/page_isolation.c code I mentioned below.
> >>>>>> No, we needn't fix mm/page_isolation.c in linux-5.10.y, because it had
> >>>>>> pfn_valid_within(buddy_pfn) check after __find_buddy_pfn() to prevent
> >>>>>> buddy_pfn=0.
> >>>>>> The root cause comes from __find_buddy_pfn():
> >>>>>> return page_pfn ^ (1 << order);
> >>>>> Right. But pfn_valid_within() was removed since 5.15. So your fix is
> >>>>> required for kernels between 5.15 and 5.17 (inclusive).
> >>>>>
> >>>>>> When page_pfn is the same as the order size, it will return the
> >>>>>> previous buddy not the next. That is the only exception for this
> >>>>>> algorithm, right?
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> In fact, the bug is a very long time to reproduce and is not easy to
> >>>>>> debug, so we want to contribute it to the community to prevent other
> >>>>>> guys from wasting time. Although there is no new patch at all.
> >>>>> Thanks for your reporting and sending out the patch. I really
> >>>>> appreciate it. We definitely need your inputs. Throughout the email
> >>>>> thread, I am trying to help you clarify the bug and how to fix it
> >>>>> properly:
> >>>>>
> >>>>> 1. The commit 787af64d05cd does not apply cleanly to commits
> >>>>> d9dddbf55667, meaning you cannot just cherry-pick that commit to
> >>>>> fix the issue. That is why we need your patch to fix the issue.
> >>>>> And saying it has a wrong fixes message in this patch’s git log is
> >>>>> misleading.
> >>>>>
> >>>>> 2. For kernels between 5.15 and 5.17 (inclusive), an additional fix
> >>>>> to mm/page_isolation.c is also needed, since pfn_valid_within() was
> >>>>> removed since 5.15 and the issue can appear during page isolation.
> >>>>>
> >>>>> 3. For kernels before 5.15, this patch will apply.
> >>>> Zi Yan, Guo Ren,
> >>>>
> >>>> I think we still need some imporvemnt for MASTER branch, as we discussed above, we will get an illegal buddy page if buddy_pfn is 0,
> >>>>
> >>>> within page_is_buddy(), it still use the illegal buddy page to do the check. I think in most of cases, page_is_buddy() can return false,  but it still may return true with very low probablity.
> >>> Can you elaborate more on this? What kind of page can lead to page_is_buddy()
> >>> returning true? You said it is buddy_pfn is 0, but if the page is reserved,
> >>> if (!page_is_guard(buddy) && !PageBuddy(buddy)) should return false.
> >>> Maybe show us the dump_page() that offending page.
> >>>
> >>> Thanks.
> >>
> >> Let‘s take the issue we met on RISC-V arch for example,
> >>
> >> pfn_base is 512 as we reserved 2M RAM for opensbi, mem_map's value is 0xffffffe07e205000, which is the page address of PFN 512.
> >>
> >> __find_buddy_pfn() returned 0 for PFN 0x2000 with order 0xd.
> >> We know PFN 0 is not a valid pfn for buddy system, because 512 is the first PFN for buddy system.
> >>
> >> Then it use below code to get buddy page with buddy_pfn 0:
> >> buddy = page + (buddy_pfn - pfn);
> >> So buddy page address is:
> >> 0xffffffe07e1fe000 = (struct page*)0xffffffe07e26e000 + (0 - 0x2000)
> >>
> >> we can know this buddy page's address is less than mem_map(0xffffffe07e1fe000 < 0xffffffe07e205000),
> >> actually 0xffffffe07e1fe000 is not a valid page's address. If we use 0xffffffe07e1fe000
> >> as the page's address to extract the value of a member in 'struct page', we may get an uncertain value.
> >> That's why I say page_is_buddy() may return true with very low probablity.
> >>
> >> So I think we need to add the code the verify buddy_pfn in the first place:
> >>      pfn_valid(buddy_pfn)
> >>
> >
> > +DavidH on how memory section works.
> >
> > This 2MB RAM reservation does not sound right to me. How does it work in sparsemem?
> > RISC-V has SECTION_SIZE_BITS=27, i.e., 128MB a section. All pages within
> > a section should have their corresponding struct page (mem_map). So in this case,
> > the first 2MB pages should have mem_map and can be marked as PageReserved. As a
> > result, page_is_buddy() will return false.
Actually, we had a patch to fix that, have a look:
https://lore.kernel.org/linux-riscv/20211123015717.542631-2-guoren@xxxxxxxxxx/
What do you think of the above patch?

A lot of arch maintainers do not recognize that the buddy system has
an implied limitation that the start of the phy ram address must align
with (1 << MAX_ORDER-1).

>
> Yes. Unless there is a BUG :)
>
> init_unavailable_range() is supposed to initialize the memap of
> unavailable ranges and mark it reserved.
>
> I wonder if we're missing a case in memmap_init(), to also initialize
> holes at the beginning of a section, before RAM (we do handle sections
> in a special way if the end of RAM falls in the middle of a section).
>
> If it's not initialized, it might contain garbage.
>
> --
> Thanks,
>
> David / dhildenb
>


--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/