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/