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. > >> >>> >>>> >>>>> 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