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Mon, 15 Aug 2022 13:35:51 GMT Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9F33811C052; Mon, 15 Aug 2022 13:35:51 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 45B6511C04C; Mon, 15 Aug 2022 13:35:51 +0000 (GMT) Received: from thinkpad (unknown [9.171.38.199]) by d06av25.portsmouth.uk.ibm.com (Postfix) with SMTP; Mon, 15 Aug 2022 13:35:51 +0000 (GMT) Date: Mon, 15 Aug 2022 15:35:49 +0200 From: Gerald Schaefer To: David Hildenbrand Cc: Mike Kravetz , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, linux-s390@vger.kernel.org Subject: Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings Message-ID: <20220815153549.0288a9c6@thinkpad> In-Reply-To: References: <20220811103435.188481-1-david@redhat.com> <20220811103435.188481-3-david@redhat.com> X-Mailer: Claws Mail 4.1.0 (GTK 3.24.34; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; 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dmarc=pass (policy=none) header.from=ibm.com X-Stat-Signature: oy7cd8kdikzsbfzmhkdm7e7wr1zwgbi7 X-HE-Tag: 1660570558-713170 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, 11 Aug 2022 11:59:09 -0700 Mike Kravetz wrote: > On 08/11/22 12:34, David Hildenbrand wrote: > > If we ever get a write-fault on a write-protected page in a shared mapping, > > we'd be in trouble (again). Instead, we can simply map the page writable. > > > > > > > Reason is that uffd-wp doesn't clear the uffd-wp PTE bit when > > unregistering and consequently keeps the PTE writeprotected. Reason for > > this is to avoid the additional overhead when unregistering. Note > > that this is the case also for !hugetlb and that we will end up with > > writable PTEs that still have the uffd-wp PTE bit set once we return > > from hugetlb_wp(). I'm not touching the uffd-wp PTE bit for now, because it > > seems to be a generic thing -- wp_page_reuse() also doesn't clear it. > > > > VM_MAYSHARE handling in hugetlb_fault() for FAULT_FLAG_WRITE > > indicates that MAP_SHARED handling was at least envisioned, but could never > > have worked as expected. > > > > While at it, make sure that we never end up in hugetlb_wp() on write > > faults without VM_WRITE, because we don't support maybe_mkwrite() > > semantics as commonly used in the !hugetlb case -- for example, in > > wp_page_reuse(). > > Nit, > to me 'make sure that we never end up in hugetlb_wp()' implies that > we would check for condition in callers as opposed to first thing in > hugetlb_wp(). However, I am OK with description as it. Is that new WARN_ON_ONCE() in hugetlb_wp() meant to indicate a real bug? It is triggered by libhugetlbfs testcase "HUGETLB_ELFMAP=R linkhuge_rw" (at least on s390), and crashes our CI, because it runs with panic_on_warn enabled. Not sure if this means that we have bug elsewhere, allowing us to get to the WARN in hugetlb_wp().