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b=WYg7oONsuX2Brvgwjitwk77p506v8el7XNP2buJRtdOxkH5PrcmxYad/4+siEEvi+ ngSsM83nqgyT19k3ROzH+GFDDT5o/PdfUBmLFewVJst0NyLUX/qVomM71taDAJmPaq qLRAesXU7j0fYjZpY71Ek+ECQFOynvtixDxkRJcrajEscwc/yi09TElybQ69+x6HlN eeBXN5dBQHJ4QARAv6jl96Oomj3R74GNuK59qp+vsBFi2/iL+5lahu1rYeQGYcZ77W n28Lu+D73MJvHHClLynmaxonmzIfYJ3AFSgBZ1uJOFvvzH5gExH+hhngKNOB5+JwHv 7MXXCtl99O6IA== Date: Mon, 3 Aug 2026 16:32:27 +0100 From: "Lorenzo Stoakes (ARM)" To: Barry Song Cc: akpm@linux-foundation.org, linux-mm@kvack.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, surenb@google.com, liam@infradead.org, vbabka@kernel.org, shakeel.butt@linux.dev, david@kernel.org, linux-kernel@vger.kernel.org, zhanghongru@xiaomi.com, willy@infradead.org, zhangbo56@xiaomi.com Subject: Re: [RFC PATCH v1 2/2] arm64/mm: use VMA lock for kernel faults on user addresses Message-ID: References: <20260802074018.73887-1-baohua@kernel.org> <20260802074018.73887-3-baohua@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: C5A6B1A000E X-Stat-Signature: e87nahupwdus3qxyti3x9gdkks63naye X-Rspam-User: X-HE-Tag: 1785771165-685541 X-HE-Meta: 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 UlS0at2T mDKST1CEBpwug/AnIqFQomxfF0CbTMvYgm56f4pZyOGPxqjVaYeCKEcsnJiTE4Pf9w8ne0XMOrsr4Z1U526m+i9nsojc0p5NSHGMM3Mzej6EuojwbA4P8icwq7+Q/J0NtiusVhcpQl3UdH2Pc6kXaynzp3k3rKPsHRUzwpZ3OI3bleqLoMq0/NXb6eDNzg7bYO7XySqakrNIw73VCSlxURKi5LDPd2U07ySENrIfeqSUWI6nP7O/3TMnThnTnqClJ5ojfJIMd9jyCZgx0m3HpGG/WisIla//k4tLV1jdChXDE/NTw3noCuB9ZCV6OfbbKhPTqi96eCMpOV97Avw7JC2di2J30AewrmR1zWLqaGRm6OOU+da0YpvvaTyksTz3xyme4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Aug 02, 2026 at 04:49:04PM +0800, Barry Song wrote: > On Sun, Aug 2, 2026 at 3:40 PM Barry Song (Xiaomi) wrote: > > > > Use the VMA lock for kernel faults on user addresses. This also > > makes the existing code below meaningful: > > > > /* Quick path to respond to signals */ > > if (fault_signal_pending(fault, regs)) { > > if (!user_mode(regs)) > > goto no_context; > > return 0; > > } > > > > Right now, the code above is dead because !user_mode always > > takes the mmap_lock path. > > > > Co-developed-by: Bo Zhang > > Signed-off-by: Bo Zhang > > Signed-off-by: Barry Song (Xiaomi) > > --- > > arch/arm64/mm/fault.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c > > index 85e23388f9bb..241f1ab07ab3 100644 > > --- a/arch/arm64/mm/fault.c > > +++ b/arch/arm64/mm/fault.c > > @@ -607,6 +607,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr, > > unsigned int mm_flags = FAULT_FLAG_DEFAULT; > > unsigned long addr = untagged_addr(far); > > struct vm_area_struct *vma; > > + bool uaccess = false; > > int si_code; > > int pkey = -1; > > > > @@ -663,6 +664,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr, > > if (!insn_may_access_user(regs->pc, esr)) > > die_kernel_fault("access to user memory outside uaccess routines", > > addr, esr, regs); > > + uaccess = true; > > https://sashiko.dev/#/patchset/20260802074018.73887-1-baohua%40kernel.org > > "Does this conditional bypass translation faults? Because this block is > guarded by: > if (is_ttbr0_addr(addr) && is_el1_permission_fault(addr, esr, regs)) { > it appears uaccess is only set for permission faults like CoW or PAN > violations. > If a kernel uaccess routine accesses an unmapped user address, a translation > fault occurs, making is_el1_permission_fault() evaluate to false. > Would this cause demand paging in uaccess routines to fall back to the slow > lock_mmap path, missing the intended optimization?" > > Good catch! I should have only modified a single line. Then > unmapped PTEs would also benefit from the VMA lock: > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c > index 85e23388f9bb..e1b406d667aa 100644 > --- a/arch/arm64/mm/fault.c > +++ b/arch/arm64/mm/fault.c > @@ -674,7 +674,7 @@ static int __kprobes do_page_fault(unsigned long > far, unsigned long esr, > > perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr); > > - if (!(mm_flags & FAULT_FLAG_USER)) > + if (!(mm_flags & FAULT_FLAG_USER) && !is_ttbr0_addr(addr)) > goto lock_mmap; I mean honestly it feels like, with the permanent VMA flags change, that maybe this really needs to wait for Matthew's series? There's duplication here too of course :) But it begs the question of every other arch that does VMA page faulting. So feels better suited as part of that change I think? Maybe Matthew has thoughts. > > vma = lock_vma_under_rcu(mm, addr); -- Cheers, Lorenzo