linux-kernel - Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode

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Date:   Wed, 24 Feb 2021 16:24:58 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Koutný <mkoutny@...e.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Xu <peterx@...hat.com>, Shaohua Li <shli@...com>,
        Shawn Anastasio <shawn@...stas.io>,
        Steven Rostedt <rostedt@...dmis.org>,
        Steven Price <steven.price@....com>,
        Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Adam Ruprecht <ruprecht@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode

On 2/18/21 4:48 PM, Axel Rasmussen wrote:
<snip>
> @@ -401,8 +398,10 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>  
>  	BUG_ON(ctx->mm != mm);
>  
> -	VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
> -	VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
> +	/* Any unrecognized flag is a bug. */
> +	VM_BUG_ON(reason & ~__VM_UFFD_FLAGS);
> +	/* 0 or > 1 flags set is a bug; we expect exactly 1. */
> +	VM_BUG_ON(!reason || !!(reason & (reason - 1)));

I may be confused, but that seems to be checking for a flag value of 1
as opposed to one flag being set?

>  
>  	if (ctx->features & UFFD_FEATURE_SIGBUS)
>  		goto out;
<snip>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3bfba75f6cbd..0388107da4b1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4352,6 +4352,38 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
>  				VM_FAULT_SET_HINDEX(hstate_index(h));
>  			goto backout_unlocked;
>  		}
> +
> +		/* Check for page in userfault range. */
> +		if (userfaultfd_minor(vma)) {
> +			u32 hash;
> +			struct vm_fault vmf = {
> +				.vma = vma,
> +				.address = haddr,
> +				.flags = flags,
> +				/*
> +				 * Hard to debug if it ends up being used by a
> +				 * callee that assumes something about the
> +				 * other uninitialized fields... same as in
> +				 * memory.c
> +				 */
> +			};
> +
> +			unlock_page(page);
> +
> +			/*
> +			 * hugetlb_fault_mutex and i_mmap_rwsem must be dropped
> +			 * before handling userfault.  Reacquire after handling
> +			 * fault to make calling code simpler.
> +			 */
> +
> +			hash = hugetlb_fault_mutex_hash(mapping, idx);
> +			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> +			i_mmap_unlock_read(mapping);
> +			ret = handle_userfault(&vmf, VM_UFFD_MINOR);
> +			i_mmap_lock_read(mapping);
> +			mutex_lock(&hugetlb_fault_mutex_table[hash]);
> +			goto out;
> +		}
>  	}
>  
>  	/*
> 

I'm good with the hugetlb.c changes.  Since this in nearly identical to
the other handle_userfault() in this routine, it might be good to create
a common wrapper.  But, that is not required.
-- 
Mike Kravetz