在 2022/6/16 下午11:04, David Hildenbrand 写道:
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@nvidia.com wrote: >> On 13 Jun 2022, at 12:32, Guo Ren wrote: >> >>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan ziy@nvidia.com 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:
your patch to stable tree prior to 5.15 and
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:
- 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.
- 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.
- 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.
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 for the comments, I will check it for RISC-V arch.