Re: [PATCH v3 9/9] userfaultfd/selftests: add test exercising minor fault handling

From: Peter Xu <peterx@redhat.com>
To: Axel Rasmussen <axelrasmussen@google.com>
Cc: "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 Kravetz" <mike.kravetz@oracle.com>,
	"Mike Rapoport" <rppt@linux.vnet.ibm.com>,
	"Nicholas Piggin" <npiggin@gmail.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>,
	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>,
	"Oliver Upton" <oupton@google.com>
Subject: Re: [PATCH v3 9/9] userfaultfd/selftests: add test exercising minor fault handling
Date: Mon, 1 Feb 2021 14:33:34 -0500	[thread overview]
Message-ID: <20210201193334.GH260413@xz-x1> (raw)
In-Reply-To: <20210128224819.2651899-10-axelrasmussen@google.com>

On Thu, Jan 28, 2021 at 02:48:19PM -0800, Axel Rasmussen wrote:
> Fix a dormant bug in userfaultfd_events_test(), where we did
> `return faulting_process(0)` instead of `exit(faulting_process(0))`.
> This caused the forked process to keep running, trying to execute any
> further test cases after the events test in parallel with the "real"
> process.
> 
> Add a simple test case which exercises minor faults. In short, it does
> the following:
> 
> 1. "Sets up" an area (area_dst) and a second shared mapping to the same
>    underlying pages (area_dst_alias).
> 
> 2. Register one of these areas with userfaultfd, in minor fault mode.
> 
> 3. Start a second thread to handle any minor faults.
> 
> 4. Populate the underlying pages with the non-UFFD-registered side of
>    the mapping. Basically, memset() each page with some arbitrary
>    contents.
> 
> 5. Then, using the UFFD-registered mapping, read all of the page
>    contents, asserting that the contents match expectations (we expect
>    the minor fault handling thread can modify the page contents before
>    resolving the fault).
> 
> The minor fault handling thread, upon receiving an event, flips all the
> bits (~) in that page, just to prove that it can modify it in some
> arbitrary way. Then it issues a UFFDIO_CONTINUE ioctl, to setup the
> mapping and resolve the fault. The reading thread should wake up and see
> this modification.
> 
> Currently the minor fault test is only enabled in hugetlb_shared mode,
> as this is the only configuration the kernel feature supports.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu