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

From: Mike Kravetz <mike.kravetz@oracle.com>
To: "Axel Rasmussen" <axelrasmussen@google.com>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Alexey Dobriyan" <adobriyan@gmail.com>,
	"Andrea Arcangeli" <aarcange@redhat.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Anshuman Khandual" <anshuman.khandual@arm.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Chinwen Chang" <chinwen.chang@mediatek.com>,
	"Huang Ying" <ying.huang@intel.com>,
	"Ingo Molnar" <mingo@redhat.com>, "Jann Horn" <jannh@google.com>,
	"Jerome Glisse" <jglisse@redhat.com>,
	"Lokesh Gidra" <lokeshgidra@google.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Michal Koutný" <mkoutny@suse.com>,
	"Michel Lespinasse" <walken@google.com>,
	"Mike Rapoport" <rppt@linux.vnet.ibm.com>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Peter Xu" <peterx@redhat.com>, "Shaohua Li" <shli@fb.com>,
	"Shawn Anastasio" <shawn@anastas.io>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Steven Price" <steven.price@arm.com>,
	"Vlastimil Babka" <vbabka@suse.cz>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, Adam Ruprecht <ruprecht@google.com>,
	Cannon Matthews <cannonmatthews@google.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Mina Almasry <almasrymina@google.com>,
	Oliver Upton <oupton@google.com>
Subject: Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode
Date: Wed, 24 Feb 2021 16:24:58 -0800	[thread overview]
Message-ID: <6aefd704-f720-35dc-d71c-da9840dc93a6@oracle.com> (raw)
In-Reply-To: <20210219004824.2899045-2-axelrasmussen@google.com>

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