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Date: Tue, 19 Jul 2011 16:24:32 +0800 From: Shan Hai <haishan.bai@...il.com> To: Benjamin Herrenschmidt <benh@...nel.crashing.org> CC: Peter Zijlstra <peterz@...radead.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, paulus@...ba.org, tglx@...utronix.de, walken@...gle.com, dhowells@...hat.com, cmetcalf@...era.com, tony.luck@...el.com, akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org Subject: Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young On 07/19/2011 03:46 PM, Benjamin Herrenschmidt wrote: > On Tue, 2011-07-19 at 13:38 +0800, Shan Hai wrote: > >> What you said is another path, that is futex_wake_op(), >> but what about futex_lock_pi in which my test case failed? >> your patch will call handle_mm_fault on every futex contention >> in the futex_lock_pi path. >> >> futex_lock_pi() >> ret = futex_lock_pi_atomic(uaddr, hb,&q.key,&q.pi_state, current, 0); >> case -EFAULT: >> goto uaddr_faulted; >> >> ... >> uaddr_faulted: >> ret = fault_in_user_writeable(uaddr); > Euh ... and how do we get to uaddr_faulted ? You may have missed the > return statement right before it :-) > > From what I can tell we only get there as a result of -EFAULT from > futex_lock_pi_atomic() which is exactly the case we are trying to > cover. > Got it, if the fault_in_user_writeable() is designed to catch the exact same write permission fault problem we discuss here, so your patch fixed that very nicely, we should fixup it by directly calling handle_mm_fault like what you did because we are for sure to know what just happened(permission violation), its not necessary to check what's happened by calling gup-->follow_page, and further the follow_page failed to report the fault :-) Thanks Shan Hai > .../... >>>> "[PATCH 1/1] Fixup write permission of TLB on powerpc e500 core"? >>>> (I will fix the stupid errors in my original patch if the concept >>>> is acceptable) >>>> in this way we could decrease the overhead of handle_mm_fault >>>> in the path which does not need write permission fixup. >>> Which overhead ? gup does handle_mm_fault() as well if needed. >> it does it *if needed*, and this requirement is rare in my opinion. > And how does gup figure out of it's needed ? By walking down the page > tables in follow_page... what does handle_mm_fault do ? walk down the > page tables... > > The main (if not the only) relevant difference here, is going to be the > spurious fault TLB invaliate for writes ... which is a nop on x86.... > and needed in all the cases we care about (and if it's not needed, then > it's up to the arch to nop it out, we should probably do it on powerpc > too ... but that's un unrelated discussion). > > Cheers, > Ben. > >> Thanks >> Shan Hai >> >>> What I do is I replace what is arguably an abuse of gup() in the case >>> where a fixup -is- needed with a dedicated function designed to perform >>> the said fixup ... and do it properly which gup() didn't :-) >>> >>> Cheers, >>> Ben. >>> >>>> Thanks >>>> Shan Hai >>>>> Cheers, >>>>> Ben. >>>>> >>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>>>> index 9670f71..1036614 100644 >>>>> --- a/include/linux/mm.h >>>>> +++ b/include/linux/mm.h >>>>> @@ -985,6 +985,8 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm, >>>>> int get_user_pages_fast(unsigned long start, int nr_pages, int write, >>>>> struct page **pages); >>>>> struct page *get_dump_page(unsigned long addr); >>>>> +extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm, >>>>> + unsigned long address, unsigned int fault_flags); >>>>> >>>>> extern int try_to_release_page(struct page * page, gfp_t gfp_mask); >>>>> extern void do_invalidatepage(struct page *page, unsigned long offset); >>>>> diff --git a/kernel/futex.c b/kernel/futex.c >>>>> index fe28dc2..7a0a4ed 100644 >>>>> --- a/kernel/futex.c >>>>> +++ b/kernel/futex.c >>>>> @@ -355,8 +355,8 @@ static int fault_in_user_writeable(u32 __user *uaddr) >>>>> int ret; >>>>> >>>>> down_read(&mm->mmap_sem); >>>>> - ret = get_user_pages(current, mm, (unsigned long)uaddr, >>>>> - 1, 1, 0, NULL, NULL); >>>>> + ret = fixup_user_fault(current, mm, (unsigned long)uaddr, >>>>> + FAULT_FLAG_WRITE); >>>>> up_read(&mm->mmap_sem); >>>>> >>>>> return ret< 0 ? ret : 0; >>>>> diff --git a/mm/memory.c b/mm/memory.c >>>>> index 40b7531..b967fb0 100644 >>>>> --- a/mm/memory.c >>>>> +++ b/mm/memory.c >>>>> @@ -1815,7 +1815,64 @@ next_page: >>>>> } >>>>> EXPORT_SYMBOL(__get_user_pages); >>>>> >>>>> -/** >>>>> +/* >>>>> + * fixup_user_fault() - manually resolve a user page fault >>>>> + * @tsk: the task_struct to use for page fault accounting, or >>>>> + * NULL if faults are not to be recorded. >>>>> + * @mm: mm_struct of target mm >>>>> + * @address: user address >>>>> + * @fault_flags:flags to pass down to handle_mm_fault() >>>>> + * >>>>> + * This is meant to be called in the specific scenario where for >>>>> + * locking reasons we try to access user memory in atomic context >>>>> + * (within a pagefault_disable() section), this returns -EFAULT, >>>>> + * and we want to resolve the user fault before trying again. >>>>> + * >>>>> + * Typically this is meant to be used by the futex code. >>>>> + * >>>>> + * The main difference with get_user_pages() is that this function >>>>> + * will unconditionally call handle_mm_fault() which will in turn >>>>> + * perform all the necessary SW fixup of the dirty and young bits >>>>> + * in the PTE, while handle_mm_fault() only guarantees to update >>>>> + * these in the struct page. >>>>> + * >>>>> + * This is important for some architectures where those bits also >>>>> + * gate the access permission to the page because their are >>>>> + * maintained in software. On such architecture, gup() will not >>>>> + * be enough to make a subsequent access succeed. >>>>> + * >>>>> + * This should be called with the mm_sem held for read. >>>>> + */ >>>>> +int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm, >>>>> + unsigned long address, unsigned int fault_flags) >>>>> +{ >>>>> + struct vm_area_struct *vma; >>>>> + int ret; >>>>> + >>>>> + vma = find_extend_vma(mm, address); >>>>> + if (!vma || address< vma->vm_start) >>>>> + return -EFAULT; >>>>> + >>>>> + ret = handle_mm_fault(mm, vma, address, fault_flags); >>>>> + if (ret& VM_FAULT_ERROR) { >>>>> + if (ret& VM_FAULT_OOM) >>>>> + return -ENOMEM; >>>>> + if (ret& (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE)) >>>>> + return -EHWPOISON; >>>>> + if (ret& VM_FAULT_SIGBUS) >>>>> + return -EFAULT; >>>>> + BUG(); >>>>> + } >>>>> + if (tsk) { >>>>> + if (ret& VM_FAULT_MAJOR) >>>>> + tsk->maj_flt++; >>>>> + else >>>>> + tsk->min_flt++; >>>>> + } >>>>> + return 0; >>>>> +} >>>>> + >>>>> +/* >>>>> * get_user_pages() - pin user pages in memory >>>>> * @tsk: the task_struct to use for page fault accounting, or >>>>> * NULL if faults are not to be recorded. >>>>> >>>>> >>>> -- >>>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >>>> the body of a message to majordomo@...r.kernel.org >>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>>> Please read the FAQ at http://www.tux.org/lkml/ >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@...r.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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