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Message-ID: <4E25185F.1050605@gmail.com>
Date: Tue, 19 Jul 2011 13:38:39 +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 01:24 PM, Benjamin Herrenschmidt wrote:
> On Tue, 2011-07-19 at 13:17 +0800, Shan Hai wrote:
>
>> The patch works, but I have certain confusions,
>> - Do we want to handle_mm_fault on each futex_lock_pi
>> even though in most cases there is no write permission
>> fixup's needed?
> Don't we only ever call this when futex_atomic_op_inuser() failed ?
> Which means a fixup -is- needed .... The fast path is still there.
>
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);
>> - How about let the archs do their own write permission
>> fixup as what I did in my original
> Why ? This is generic and will fix all archs at once with generic code
> which is a significant improvement in my book and a lot more
> maintainable :-)
>
If the overhead in the futex_lock_pi path is not considerable yes fix it up
generally is nice :-)
>> "[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.
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.
>>>
>>>
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