Re: [PATCH 6/6] mm: proc: Avoid fullmm flush for young/dirty bit toggling

[Yu Zhao <yuzhao@google.com>]

On Mon, Nov 23, 2020 at 06:35:55PM +0000, Will Deacon wrote:
> On Fri, Nov 20, 2020 at 01:40:05PM -0700, Yu Zhao wrote:
> > On Fri, Nov 20, 2020 at 02:35:57PM +0000, Will Deacon wrote:
> > > clear_refs_write() uses the 'fullmm' API for invalidating TLBs after
> > > updating the page-tables for the current mm. However, since the mm is not
> > > being freed, this can result in stale TLB entries on architectures which
> > > elide 'fullmm' invalidation.
> > > 
> > > Ensure that TLB invalidation is performed after updating soft-dirty
> > > entries via clear_refs_write() by using the non-fullmm API to MMU gather.
> > > 
> > > Signed-off-by: Will Deacon <will@kernel.org>
> > > ---
> > >  fs/proc/task_mmu.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > index a76d339b5754..316af047f1aa 100644
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -1238,7 +1238,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
> > >  			count = -EINTR;
> > >  			goto out_mm;
> > >  		}
> > > -		tlb_gather_mmu_fullmm(&tlb, mm);
> > > +		tlb_gather_mmu(&tlb, mm, 0, TASK_SIZE);
> > 
> > Let's assume my reply to patch 4 is wrong, and therefore we still need
> > tlb_gather/finish_mmu() here. But then wouldn't this change deprive
> > architectures other than ARM the opportunity to optimize based on the
> > fact it's a full-mm flush?

I double checked my conclusion on patch 4, and aside from a couple
of typos, it still seems correct after the weekend.

> Only for the soft-dirty case, but I think TLB invalidation is required
> there because we are write-protecting the entries and I don't see any
> mechanism to handle lazy invalidation for that (compared with the aging
> case, which is handled via pte_accessible()).

The lazy invalidation for that is done when we write-protect a page,
not an individual PTE. When we do so, our decision is based on both
the dirty bit and the writable bit on each PTE mapping this page. So
we only need to make sure we don't lose both on a PTE. And we don't
here.

> Furthermore, If we decide that we can relax the TLB invalidation
> requirements here, then I'd much rather than was done deliberately, rather
> than as an accidental side-effect of another commit (since I think the
> current behaviour was a consequence of 7a30df49f63a).

Nope. tlb_gather/finish_mmu() should be added by b3a81d0841a9
("mm: fix KSM data corruption") in the first place.

> > It seems to me ARM's interpretation of tlb->fullmm is a special case,
> > not the other way around.
> 
> Although I agree that this is subtle and error-prone (which is why I'm
> trying to make the API more explicit here), it _is_ documented clearly
> in asm-generic/tlb.h:
> 
>  *  - mmu_gather::fullmm
>  *
>  *    A flag set by tlb_gather_mmu() to indicate we're going to free
>  *    the entire mm; this allows a number of optimizations.
>  *
>  *    - We can ignore tlb_{start,end}_vma(); because we don't
>  *      care about ranges. Everything will be shot down.
>  *
>  *    - (RISC) architectures that use ASIDs can cycle to a new ASID
>  *      and delay the invalidation until ASID space runs out.

I'd leave the original tlb_gather/finish_mmu() for the first case and
add a new API for the second case, the special case that only applies
to exit_mmap()). This way we won't change any existing behaviors on
other architectures, which seems important to me.

Additional cleanups to tlb_gather/finish_mmu() come thereafter.