linux-kernel - Re: [PATCH rfc -next 01/10] mm: add a generic VMA lock-based page fault handler

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Date:   Thu, 13 Jul 2023 17:15:17 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        surenb@...gle.com, Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, loongarch@...ts.linux.dev,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH rfc -next 01/10] mm: add a generic VMA lock-based page
 fault handler

> +int try_vma_locked_page_fault(struct vm_locked_fault *vmlf, vm_fault_t *ret)
> +{
> +	struct vm_area_struct *vma;
> +	vm_fault_t fault;


On Thu, Jul 13, 2023 at 05:53:29PM +0800, Kefeng Wang wrote:
> +#define VM_LOCKED_FAULT_INIT(_name, _mm, _address, _fault_flags, _vm_flags, _regs, _fault_code) \
> +	_name.mm		= _mm;			\
> +	_name.address		= _address;		\
> +	_name.fault_flags	= _fault_flags;		\
> +	_name.vm_flags		= _vm_flags;		\
> +	_name.regs		= _regs;		\
> +	_name.fault_code	= _fault_code

More consolidated code is a good idea; no question.  But I don't think
this is the right way to do it.

> +int __weak arch_vma_check_access(struct vm_area_struct *vma,
> +				 struct vm_locked_fault *vmlf);

This should be:

#ifndef vma_check_access
bool vma_check_access(struct vm_area_struct *vma, )
{
	return (vma->vm_flags & vm_flags) == 0;
}
#endif

and then arches which want to do something different can just define
vma_check_access.

> +int try_vma_locked_page_fault(struct vm_locked_fault *vmlf, vm_fault_t *ret)
> +{
> +	struct vm_area_struct *vma;
> +	vm_fault_t fault;

Declaring the vmf in this function and then copying it back is just wrong.
We need to declare vm_fault_t earlier (in the arch fault handler) and
pass it in.  I don't think that creating struct vm_locked_fault is the
right idea either.

> +	if (!(vmlf->fault_flags & FAULT_FLAG_USER))
> +		return -EINVAL;
> +
> +	vma = lock_vma_under_rcu(vmlf->mm, vmlf->address);
> +	if (!vma)
> +		return -EINVAL;
> +
> +	if (arch_vma_check_access(vma, vmlf)) {
> +		vma_end_read(vma);
> +		return -EINVAL;
> +	}
> +
> +	fault = handle_mm_fault(vma, vmlf->address,
> +				vmlf->fault_flags | FAULT_FLAG_VMA_LOCK,
> +				vmlf->regs);
> +	*ret = fault;
> +
> +	if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> +		vma_end_read(vma);
> +
> +	if ((fault & VM_FAULT_RETRY))
> +		count_vm_vma_lock_event(VMA_LOCK_RETRY);
> +	else
> +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +
> +	return 0;
> +}
> +
>  #endif /* CONFIG_PER_VMA_LOCK */
>  
>  #ifndef __PAGETABLE_P4D_FOLDED
> -- 
> 2.27.0
> 
>