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[174.95.215.133]) by smtp.gmail.com with ESMTPSA id k24sm353066qtp.35.2020.10.30.15.52.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Oct 2020 15:52:51 -0700 (PDT) Date: Fri, 30 Oct 2020 18:52:50 -0400 From: Peter Xu To: Jason Gunthorpe Cc: linux-kernel@vger.kernel.org, Linus Torvalds , Andrea Arcangeli , Andrew Morton , "Aneesh Kumar K.V" , Christoph Hellwig , Hugh Dickins , Jan Kara , Jann Horn , John Hubbard , Kirill Shutemov , Kirill Tkhai , Leon Romanovsky , Linux-MM , Michal Hocko , Oleg Nesterov Subject: Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Message-ID: <20201030225250.GB6357@xz-x1> References: <0-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com> <2-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <2-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Jason, I think majorly the patch looks good to me, but I have a few pure questions majorly not directly related to the patch itself, but around the contexts. Since I _feel_ like there'll be a new version to update the comments below, maybe I can still ask aloud... Please bare with me. :) On Fri, Oct 30, 2020 at 11:46:21AM -0300, Jason Gunthorpe wrote: > Slow GUP is safe against this race because copy_page_range() is only > called while holding the exclusive side of the mmap_lock on the src > mm_struct. Pure question: I understand that this patch requires this, but... Could anyone remind me why read lock of mmap_sem is not enough for fork() before this one? > > Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()") > Suggested-by: Linus Torvalds > Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com > Reviewed-by: John Hubbard > Signed-off-by: Jason Gunthorpe > --- > arch/x86/kernel/tboot.c | 1 + > drivers/firmware/efi/efi.c | 1 + > include/linux/mm_types.h | 7 +++++++ > kernel/fork.c | 1 + > mm/gup.c | 19 +++++++++++++++++++ > mm/init-mm.c | 1 + > mm/memory.c | 10 +++++++++- > 7 files changed, 39 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c > index 992fb1415c0f1f..6a2f542d9588a4 100644 > --- a/arch/x86/kernel/tboot.c > +++ b/arch/x86/kernel/tboot.c > @@ -93,6 +93,7 @@ static struct mm_struct tboot_mm = { > .pgd = swapper_pg_dir, > .mm_users = ATOMIC_INIT(2), > .mm_count = ATOMIC_INIT(1), > + .write_protect_seq = SEQCNT_ZERO(tboot_mm.write_protect_seq), > MMAP_LOCK_INITIALIZER(init_mm) > .page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock), > .mmlist = LIST_HEAD_INIT(init_mm.mmlist), > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c > index 5e5480a0a32d7d..2520f6e05f4d44 100644 > --- a/drivers/firmware/efi/efi.c > +++ b/drivers/firmware/efi/efi.c > @@ -57,6 +57,7 @@ struct mm_struct efi_mm = { > .mm_rb = RB_ROOT, > .mm_users = ATOMIC_INIT(2), > .mm_count = ATOMIC_INIT(1), > + .write_protect_seq = SEQCNT_ZERO(efi_mm.write_protect_seq), > MMAP_LOCK_INITIALIZER(efi_mm) > .page_table_lock = __SPIN_LOCK_UNLOCKED(efi_mm.page_table_lock), > .mmlist = LIST_HEAD_INIT(efi_mm.mmlist), Another pure question: I'm just curious how you find all the statically definied mm_structs, and to make sure all of them are covered (just in case un-initialized seqcount could fail strangely). Actually I'm thinking whether we should have one place to keep all the init vars for all the statically definied mm_structs, so we don't need to find them everytime, but only change that one place. > diff --git a/mm/memory.c b/mm/memory.c > index c48f8df6e50268..294c2c3c4fe00d 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -1171,6 +1171,12 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) > mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE, > 0, src_vma, src_mm, addr, end); > mmu_notifier_invalidate_range_start(&range); > + /* > + * The read side doesn't spin, it goes to the mmap_lock, so the > + * raw version is used to avoid disabling preemption here > + */ > + mmap_assert_write_locked(src_mm); > + raw_write_seqcount_t_begin(&src_mm->write_protect_seq); Would raw_write_seqcount_begin() be better here? My understanding is that we used raw_write_seqcount_t_begin() because we're with spin lock so assuming we disabled preemption already. However I'm thinking whether raw_write_seqcount_begin() would be even better to guarantee that. I have no idea of how the rt kernel merging topic, but if rt kernel merged into mainline then IIUC preemption is allowed here (since pgtable spin lock should be rt_spin_lock, not raw spin locks). An even further pure question on __seqcount_preemptible() (feel free to ignore this question!): I saw that __seqcount_preemptible() seems to have been constantly defined as "return false". Not sure what happened there.. Thanks, -- Peter Xu