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Tue, 12 Aug 2025 06:47:05 -0700 (PDT) From: Yongting Lin To: anthony.yznaga@oracle.com Cc: akpm@linux-foundation.org, andreyknvl@gmail.com, arnd@arndb.de, brauner@kernel.org, catalin.marinas@arm.com, dave.hansen@intel.com, david@redhat.com, ebiederm@xmission.com, khalid@kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, luto@kernel.org, markhemm@googlemail.com, maz@kernel.org, mhiramat@kernel.org, neilb@suse.de, pcc@google.com, rostedt@goodmis.org, vasily.averin@linux.dev, viro@zeniv.linux.org.uk, willy@infradead.org, xhao@linux.alibaba.com Subject: Re: [PATCH v2 13/20] x86/mm: enable page table sharing Date: Tue, 12 Aug 2025 21:46:55 +0800 Message-Id: <20250812134655.68614-1-linyongting@bytedance.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20250404021902.48863-14-anthony.yznaga@oracle.com> References: <20250404021902.48863-14-anthony.yznaga@oracle.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Hi, On 4/4/25 10:18 AM, Anthony Yznaga wrote: > Enable x86 support for handling page faults in an mshare region by > redirecting page faults to operate on the mshare mm_struct and vmas > contained in it. > Some permissions checks are done using vma flags in architecture-specfic > fault handling code so the actual vma needed to complete the handling > is acquired before calling handle_mm_fault(). Because of this an > ARCH_SUPPORTS_MSHARE config option is added. > > Signed-off-by: Anthony Yznaga > --- > arch/Kconfig | 3 +++ > arch/x86/Kconfig | 1 + > arch/x86/mm/fault.c | 37 ++++++++++++++++++++++++++++++++++++- > mm/Kconfig | 2 +- > 4 files changed, 41 insertions(+), 2 deletions(-) > > diff --git a/arch/Kconfig b/arch/Kconfig > index 9f6eb09ef12d..2e000fefe9b3 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -1652,6 +1652,9 @@ config HAVE_ARCH_PFN_VALID > config ARCH_SUPPORTS_DEBUG_PAGEALLOC > bool > > +config ARCH_SUPPORTS_MSHARE > + bool > + > config ARCH_SUPPORTS_PAGE_TABLE_CHECK > bool > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > index 1502fd0c3c06..1f1779decb44 100644 > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -125,6 +125,7 @@ config X86 > select ARCH_SUPPORTS_ACPI > select ARCH_SUPPORTS_ATOMIC_RMW > select ARCH_SUPPORTS_DEBUG_PAGEALLOC > + select ARCH_SUPPORTS_MSHARE if X86_64 > select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64 > select ARCH_SUPPORTS_NUMA_BALANCING if X86_64 > select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP if NR_CPUS <= 4096 > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > index 296d294142c8..49659d2f9316 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -1216,6 +1216,8 @@ void do_user_addr_fault(struct pt_regs *regs, > struct mm_struct *mm; > vm_fault_t fault; > unsigned int flags = FAULT_FLAG_DEFAULT; > + bool is_shared_vma; > + unsigned long addr; > > tsk = current; > mm = tsk->mm; > @@ -1329,6 +1331,12 @@ void do_user_addr_fault(struct pt_regs *regs, > if (!vma) > goto lock_mmap; > > + /* mshare does not support per-VMA locks yet */ > + if (vma_is_mshare(vma)) { > + vma_end_read(vma); > + goto lock_mmap; > + } > + > if (unlikely(access_error(error_code, vma))) { > bad_area_access_error(regs, error_code, address, NULL, vma); > count_vm_vma_lock_event(VMA_LOCK_SUCCESS); > @@ -1357,17 +1365,38 @@ void do_user_addr_fault(struct pt_regs *regs, > lock_mmap: > > retry: > + addr = address; > + is_shared_vma = false; > vma = lock_mm_and_find_vma(mm, address, regs); > if (unlikely(!vma)) { > bad_area_nosemaphore(regs, error_code, address); > return; > } > > + if (unlikely(vma_is_mshare(vma))) { > + fault = find_shared_vma(&vma, &addr); > + > + if (fault) { > + mmap_read_unlock(mm); > + goto done; > + } > + > + if (!vma) { > + mmap_read_unlock(mm); > + bad_area_nosemaphore(regs, error_code, address); > + return; > + } > + > + is_shared_vma = true; > + } > + > /* > * Ok, we have a good vm_area for this memory access, so > * we can handle it.. > */ > if (unlikely(access_error(error_code, vma))) { > + if (unlikely(is_shared_vma)) > + mmap_read_unlock(vma->vm_mm); > bad_area_access_error(regs, error_code, address, mm, vma); > return; > } > @@ -1385,7 +1414,11 @@ void do_user_addr_fault(struct pt_regs *regs, > * userland). The return to userland is identified whenever > * FAULT_FLAG_USER|FAULT_FLAG_KILLABLE are both set in flags. > */ > - fault = handle_mm_fault(vma, address, flags, regs); > + fault = handle_mm_fault(vma, addr, flags, regs); > + > + if (unlikely(is_shared_vma) && ((fault & VM_FAULT_COMPLETED) || > + (fault & VM_FAULT_RETRY) || fault_signal_pending(fault, regs))) > + mmap_read_unlock(mm); I was backporting these patches of mshare to 5.15 kernel and trying to do some basic tests. Then found a potential issue. Reaching here means find_shared_vma function has been executed successfully and host_mm->mmap_lock has got locked. When returned fault variable has VM_FAULT_COMPLETED or VM_FAULT_RETRY flags, or fault_signal_pending(fault, regs) takes true, there is not chance to release locks of both mm and host_mm(i.e. vma->vm_mm) in the following Snippet of Code. As a result, needs to release vma->vm_mm.mmap_lock as well. So it is supposed to be like below: - fault = handle_mm_fault(vma, address, flags, regs); + fault = handle_mm_fault(vma, addr, flags, regs); + + if (unlikely(is_shared_vma) && ((fault & VM_FAULT_COMPLETED) || + (fault & VM_FAULT_RETRY) || fault_signal_pending(fault, regs))) { + mmap_read_unlock(vma->vm_mm); + mmap_read_unlock(mm); + } > > if (fault_signal_pending(fault, regs)) { > /* > @@ -1413,6 +1446,8 @@ void do_user_addr_fault(struct pt_regs *regs, > goto retry; > } > > + if (unlikely(is_shared_vma)) > + mmap_read_unlock(vma->vm_mm); > mmap_read_unlock(mm); > done: > if (likely(!(fault & VM_FAULT_ERROR))) > diff --git a/mm/Kconfig b/mm/Kconfig > index e6c90db83d01..8a5a159457f2 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -1344,7 +1344,7 @@ config PT_RECLAIM > > config MSHARE > bool "Mshare" > - depends on MMU > + depends on MMU && ARCH_SUPPORTS_MSHARE > help > Enable msharefs: A ram-based filesystem that allows multiple > processes to share page table entries for shared pages. A file Yongting Lin.