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 wrote: >>>> On 13 Jun 2022, at 12:32, Guo Ren wrote: >>>> >>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan 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. > > I think we need to add some code to verify buddy_pfn in the first place. > > Could you give some suggestions for this idea? > >> >>>>>>> Actually, this issue is involved by commit: >>>>>>> commit d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks") >>>>>>> >>>>>>> For RISC-V arch, the first 2M is reserved for sbi, so the start PFN is 512, >>>>>>> but it got buddy PFN 0 for PFN 0x2000: >>>>>>> 0 = 0x2000 ^ (1 << 12) >>>>>>> With the illegal buddy PFN 0, it got an illegal buddy page, which caused >>>>>>> crash in __get_pfnblock_flags_mask(). >>>>>> It seems that the RISC-V arch reveals a similar bug from d9dddbf55667. >>>>>> Basically, this bug will only happen when PFN=0x2000 is merging up and >>>>>> there are some isolated pageblocks. >>>>> Not PFN=0x2000, it's PFN=0x1000, I guess. >>>>> >>>>> RISC-V's first 2MB RAM could reserve for opensbi, so it would have >>>>> riscv_pfn_base=512 and mem_map began with 512th PFN when >>>>> CONFIG_FLATMEM=y. >>>>> (Also, csky has the same issue: a non-zero pfn_base in some scenarios.) >>>>> >>>>> But __find_buddy_pfn algorithm thinks the start address is 0, it could >>>>> get 0 pfn or less than the pfn_base value. We need another check to >>>>> prevent that. >>>>> >>>>>> BTW, what does first reserved 2MB imply? All 4KB pages from first 2MB are >>>>>> set to PageReserved? >>>>>> >>>>>>> With the patch, it can avoid the calling of get_pageblock_migratetype() if >>>>>>> it isn't buddy page. >>>>>> You might miss the __find_buddy_pfn() caller in unset_migratetype_isolate() >>>>>> from mm/page_isolation.c, if you are talking about linux-5.17.y and former >>>>>> version. There, page_is_buddy() is also not called and is_migrate_isolate_page() >>>>>> is called, which calls get_pageblock_migratetype() too. >>>>>> >>>>>>> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks") >>>>>>> Cc: stable@vger.kernel.org >>>>>>> Reported-by: zjb194813@alibaba-inc.com >>>>>>> Reported-by: tianhu.hh@alibaba-inc.com >>>>>>> Signed-off-by: Xianting Tian >>>>>>> --- >>>>>>> mm/page_alloc.c | 3 +++ >>>>>>> 1 file changed, 3 insertions(+) >>>>>>> >>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>>>>> index b1caa1c6c887..5b423caa68fd 100644 >>>>>>> --- a/mm/page_alloc.c >>>>>>> +++ b/mm/page_alloc.c >>>>>>> @@ -1129,6 +1129,9 @@ static inline void __free_one_page(struct page *page, >>>>>>> >>>>>>> buddy_pfn = __find_buddy_pfn(pfn, order); >>>>>>> buddy = page + (buddy_pfn - pfn); >>>>>>> + >>>>>>> + if (!page_is_buddy(page, buddy, order)) >>>>>>> + goto done_merging; >>>>>>> buddy_mt = get_pageblock_migratetype(buddy); >>>>>>> >>>>>>> if (migratetype != buddy_mt >>>>>>> -- >>>>>>> 2.17.1 >>>>>> -- >>>>>> Best Regards, >>>>>> Yan, Zi >>>>> >>>>> >>>>> -- >>>>> Best Regards >>>>> Guo Ren >>>>> >>>>> ML: https://lore.kernel.org/linux-csky/ >>>> -- >>>> Best Regards, >>>> Yan, Zi >>> >>> >>> -- >>> Best Regards >>> Guo Ren >>> >>> ML: https://lore.kernel.org/linux-csky/ >> -- >> Best Regards, >> Yan, Zi -- Best Regards, Yan, Zi