Messages in this thread | | | Date | Sat, 16 Jul 2011 11:36:19 -0400 | From | Shan Hai <> | Subject | Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core |
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On 07/15/2011 11:24 AM, Peter Zijlstra wrote: > On Fri, 2011-07-15 at 11:18 -0400, Shan Hai wrote: > >>>> + vma = find_vma(mm, address); >>> Uhm, find_vma() needs mmap_sem, and futex_atomic_cmpxchg_inatomic() is >>> most certainly not called with that lock held. >>> >> My fault, that will be fixed in the V2 patch. > But you cannot, the function isn't called _atomic_ just for kicks, its > used while holding spinlocks. >
Yes we can do that, _atomic_ here is just atomic for cmpxchg implemented by the combination of 'lwarx' and 'stwcx.' instructions as done in the spin lock implementation, so even we hold the mmap_sem that has no impact on the _atomic_ feature of the futex_atomic_cmpxchg_inatomic().
>>>> + if (likely(vma)) { >>>> + /* only fixup present page */ >>>> + if (follow_page(vma, address, FOLL_WRITE)) { >>>> + handle_mm_fault(mm, vma, address, FAULT_FLAG_WRITE); >>> So how can this toggle your sw dirty/young tracking, that's pretty much >>> what gup(.write=1) does too! >>> >> because of the kernel read only permission of the page is transparent >> to the follow_page(), the handle_mm_fault() is not to be activated >> in the __get_use_pages(), so the gup(.write=1) could not help to fixup >> the write permission. > So why do you need the vma? Is it like I wrote earlier that you don't > have spare PTE bits and need the vma flags to see if it may become > writable? >
Need vma for the reason to call handle_mm_fault(), that's all.
> gup(.write=1) not triggering this is a serious problem though, not > something you can just paper over. I wouldn't be at all surprised to > find there's more things broken because of that.
In my opinion another solution might be check the read only for kernel feature of a page in the follow_page() on gup(.write=1) to avoid this problem on all architectures.
Thanks Shan Hai
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