linux-kernel - Re: [PATCH v9 04/24] mm: Prepare for FAULT_FLAG

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Date:   Wed, 4 Apr 2018 11:23:05 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, sergey.senozhatsky.work@...il.com,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v9 04/24] mm: Prepare for FAULT_FLAG_SPECULATIVE



On 03/04/2018 23:57, David Rientjes wrote:
> On Wed, 28 Mar 2018, Laurent Dufour wrote:
> 
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index 4d02524a7998..2f3e98edc94a 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -300,6 +300,7 @@ extern pgprot_t protection_map[16];
>>>>  #define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
>>>>  #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
>>>>  #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
>>>> +#define FAULT_FLAG_SPECULATIVE	0x200	/* Speculative fault, not holding mmap_sem */
>>>>  
>>>>  #define FAULT_FLAG_TRACE \
>>>>  	{ FAULT_FLAG_WRITE,		"WRITE" }, \
>>>
>>> I think FAULT_FLAG_SPECULATIVE should be introduced in the patch that 
>>> actually uses it.
>>
>> I think you're right, I'll move down this define in the series.
>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index e0ae4999c824..8ac241b9f370 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -2288,6 +2288,13 @@ int apply_to_page_range(struct mm_struct *mm, unsigned long addr,
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(apply_to_page_range);
>>>>  
>>>> +static bool pte_map_lock(struct vm_fault *vmf)
>>>
>>> inline?
>>
>> Agreed.
>>
> 
> Ignore this, the final form of the function after the full patchset 
> shouldn't be inline.

Indeed, I only kept as inlined the small pte_map_lock() and later
pte_spinlock() defined when CONFIG_SPECULATIVE_PAGE_FAULT is not set.

>>>> +{
>>>> +	vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
>>>> +				       vmf->address, &vmf->ptl);
>>>> +	return true;
>>>> +}
>>>> +
>>>>  /*
>>>>   * handle_pte_fault chooses page fault handler according to an entry which was
>>>>   * read non-atomically.  Before making any commitment, on those architectures
>>>> @@ -2477,6 +2484,7 @@ static int wp_page_copy(struct vm_fault *vmf)
>>>>  	const unsigned long mmun_start = vmf->address & PAGE_MASK;
>>>>  	const unsigned long mmun_end = mmun_start + PAGE_SIZE;
>>>>  	struct mem_cgroup *memcg;
>>>> +	int ret = VM_FAULT_OOM;
>>>>  
>>>>  	if (unlikely(anon_vma_prepare(vma)))
>>>>  		goto oom;
>>>> @@ -2504,7 +2512,11 @@ static int wp_page_copy(struct vm_fault *vmf)
>>>>  	/*
>>>>  	 * Re-check the pte - we dropped the lock
>>>>  	 */
>>>> -	vmf->pte = pte_offset_map_lock(mm, vmf->pmd, vmf->address, &vmf->ptl);
>>>> +	if (!pte_map_lock(vmf)) {
>>>> +		mem_cgroup_cancel_charge(new_page, memcg, false);
>>>> +		ret = VM_FAULT_RETRY;
>>>> +		goto oom_free_new;
>>>> +	}
>>>
>>> Ugh, but we aren't oom here, so maybe rename oom_free_new so that it makes 
>>> sense for return values other than VM_FAULT_OOM?
>>
>> You're right, now this label name is not correct, I'll rename it to
>> "out_free_new" and rename also the label "oom" to "out" since it is generic too
>> now.
>>
> 
> I think it would just be better to introduce a out_uncharge that handles 
> the mem_cgroup_cancel_charge() in the exit path.

Yes adding an out_uncharge label sounds good too. I'll add it and also rename
oom_* ones to out_*.

> 
> diff --git a/mm/memory.c b/mm/memory.c
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2645,9 +2645,8 @@ static int wp_page_copy(struct vm_fault *vmf)
>  	 * Re-check the pte - we dropped the lock
>  	 */
>  	if (!pte_map_lock(vmf)) {
> -		mem_cgroup_cancel_charge(new_page, memcg, false);
>  		ret = VM_FAULT_RETRY;
> -		goto oom_free_new;
> +		goto out_uncharge;
>  	}
>  	if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
>  		if (old_page) {
> @@ -2735,6 +2734,8 @@ static int wp_page_copy(struct vm_fault *vmf)
>  		put_page(old_page);
>  	}
>  	return page_copied ? VM_FAULT_WRITE : 0;
> +out_uncharge:
> +	mem_cgroup_cancel_charge(new_page, memcg, false);
>  oom_free_new:
>  	put_page(new_page);
>  oom:
>