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 <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@xxxxxxxxxxxxxx
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> ---
> 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
