Messages in this thread | | | Date | Wed, 24 Aug 2022 23:23:10 +0800 | Subject | Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE size hugetlb page | From | Baolin Wang <> |
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On 8/24/2022 11:13 PM, David Hildenbrand wrote: > On 24.08.22 17:06, Baolin Wang wrote: >> >> >> On 8/24/2022 10:33 PM, David Hildenbrand wrote: >>> On 24.08.22 16:30, Baolin Wang wrote: >>>> >>>> >>>> On 8/24/2022 7:55 PM, David Hildenbrand wrote: >>>>> On 24.08.22 11:41, Baolin Wang wrote: >>>>>> >>>>>> >>>>>> On 8/24/2022 3:31 PM, David Hildenbrand wrote: >>>>>>>>>>> >>>>>>>>>>> IMHO, these follow_huge_xxx() functions are arch-specified at first and >>>>>>>>>>> were moved into the common hugetlb.c by commit 9e5fc74c3025 ("mm: >>>>>>>>>>> hugetlb: Copy general hugetlb code from x86 to mm"), and now there are >>>>>>>>>>> still some arch-specified follow_huge_xxx() definition, for example: >>>>>>>>>>> ia64: follow_huge_addr >>>>>>>>>>> powerpc: follow_huge_pd >>>>>>>>>>> s390: follow_huge_pud >>>>>>>>>>> >>>>>>>>>>> What I mean is that follow_hugetlb_page() is a common and >>>>>>>>>>> not-arch-specified function, is it suitable to change it to be >>>>>>>>>>> arch-specified? >>>>>>>>>>> And thinking more, can we rename follow_hugetlb_page() as >>>>>>>>>>> hugetlb_page_faultin() and simplify it to only handle the page faults of >>>>>>>>>>> hugetlb like the faultin_page() for normal page? That means we can make >>>>>>>>>>> sure only follow_page_mask() can handle hugetlb. >>>>>>>>>>> >>>>>>>>> >>>>>>>>> Something like that might work, but you still have two page table walkers >>>>>>>>> for hugetlb. I like David's idea (if I understand it correctly) of >>>>>>>> >>>>>>>> What I mean is we may change the hugetlb handling like normal page: >>>>>>>> 1) use follow_page_mask() to look up a hugetlb firstly. >>>>>>>> 2) if can not get the hugetlb, then try to page fault by >>>>>>>> hugetlb_page_faultin(). >>>>>>>> 3) if page fault successed, then retry to find hugetlb by >>>>>>>> follow_page_mask(). >>>>>>> >>>>>>> That implies putting more hugetlbfs special code into generic GUP, >>>>>>> turning it even more complicated. But of course, it depends on how the >>>>>>> end result looks like. My gut feeling was that hugetlb is better handled >>>>>>> in follow_hugetlb_page() separately (just like we do with a lot of other >>>>>>> page table walkers). >>>>>> >>>>>> OK, fair enough. >>>>>> >>>>>>>> >>>>>>>> Just a rough thought, and I need more investigation for my idea and >>>>>>>> David's idea. >>>>>>>> >>>>>>>>> using follow_hugetlb_page for both cases. As noted, it will need to be >>>>>>>>> taught how to not trigger faults in the follow_page_mask case. >>>>>>>> >>>>>>>> Anyway, I also agree we need some cleanup, and firstly I think we should >>>>>>>> cleanup these arch-specified follow_huge_xxx() on some architectures >>>>>>>> which are similar with the common ones. I will look into these. >>>>>>> >>>>>>> There was a recent discussion on that, e.g.: >>>>>>> >>>>>>> https://lkml.kernel.org/r/20220818135717.609eef8a@thinkpad >>>>>> >>>>>> Thanks. >>>>>> >>>>>>> >>>>>>>> >>>>>>>> However, considering cleanup may need more investigation and >>>>>>>> refactoring, now I prefer to make these bug-fix patches of this patchset >>>>>>>> into mainline firstly, which are suitable to backport to old version to >>>>>>>> fix potential race issues. Mike and David, how do you think? Could you >>>>>>>> help to review these patches? Thanks. >>>>>>> >>>>>>> Patch #1 certainly add more special code just to handle another hugetlb >>>>>>> corner case (CONT pages), and maybe just making it all use >>>>>>> follow_hugetlb_page() would be even cleaner and less error prone. >>>>>>> >>>>>>> I agree that locking is shaky, but I'm not sure if we really want to >>>>>>> backport this to stable trees: >>>>>>> >>>>>>> https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html >>>>>>> >>>>>>> "It must fix a real bug that bothers people (not a, “This could be a >>>>>>> problem...” type thing)." >>>>>>> >>>>>>> >>>>>>> Do we actually have any instance of this being a real (and not a >>>>>>> theoretical) problem? If not, I'd rather clean it all up right away. >>>>>> >>>>>> I think this is a real problem (not theoretical), and easy to write some >>>>>> code to show the issue. For example, suppose thread A is trying to look >>>>>> up a CONT-PTE size hugetlb page under the lock, however antoher thread B >>>>>> can migrate the CONT-PTE hugetlb page at the same time, which will cause >>>>>> thread A to get an incorrect page, if thread A want to do something for >>>>>> this incorrect page, error occurs. >>>>>> >>>>>> Actually we also want to backport these fixes to the distro with old >>>>>> kernel versions to make the hugetlb more stable. Otherwise we must hit >>>>>> these issues sooner or later if the customers use CONT-PTE/PMD hugetlb. >>>>>> >>>>>> Anyway, if you and Mike still think these issues are not important >>>>>> enough to be fixed in the old versions, I can do the cleanup firstly. >>>>>> >>>>> >>>>> [asking myself which follow_page() users actually care about hugetlb, >>>>> and why we need this handling in follow_page at all] >>>>> >>>>> Which follow_page() user do we care about here? Primarily mm/migrate.c >>>>> only I assume? >>>> >>>> Right, mainly affects the move_pages() syscall I think. Yes, I can not >>>> know all of the users of the move_pages() syscall now or in the future >>>> in our data center, but like I said the move_pages() syscall + hugetlb >>>> can be a real potential stability issue. >>>> >>> >>> I wonder if we can get rid of follow_page() completely, there are not >>> too many users. Or alternatively simply make it use general GUP >>> infrastructure more clearly. We'd need something like FOLL_NOFAULT that >>> also covers "absolutely no faults". >> >> I am not sure I get your point. So you want change to use >> __get_user_pages() (or silimar wrappers) to look up a normal page or >> hugetlb instead of follow_page()? and adding a new FOLL_NOFAULT flag to >> __get_user_pages(). > > Essentially just getting rid of follow_page() completely or making it a > wrapper of __get_user_pages().
OK.
> >> >> If I understand correctly, we still need more work to move those >> arch-specified follow_huge_xxx() into follow_hugetlb_page() firstly like >> we disscussed before? Which seems not backportable too. > > I'm not sure we need all that magic in these arch specific helpers after > all. I haven't looked into the details, but I really wonder why they > handle something that follow_hugetlb_page() cannot easily handle. It all > smells like legacy cruft.
Agree, not sure if there are some historical legacy issue, need more investigation.
> >> >> I am not againt your idea, and I also agree that we should do some >> cleanup. But the point is if we need backport patches to fix this issue, >> which affects move_pages() syscall, if the answer is yes, I think my >> current fixes are suitable to backport. > > I really don't like adding more make-legacy-cruft-happy code unless > there is *real* need for it. (you could always just fix old kernels you > care about with your patches here -- do they have to be in mainline?
Not have to, but better...
> don't think so) > > But of course, it's up to Mike to decide, just my 2 cents :)
Thanks for your useful comments, and let's see what's the view of Mike :)
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