[PATCH 6/6] mm: proc: Avoid fullmm flush for young/dirty bit toggling
Yu Zhao
yuzhao at google.com
Mon Nov 23 20:13:34 EST 2020
On Mon, Nov 23, 2020 at 09:17:51PM +0000, Will Deacon wrote:
> On Mon, Nov 23, 2020 at 01:04:03PM -0700, Yu Zhao wrote:
> > 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 at 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.
>
> I still need to digest that, but I would prefer that we restore the
> invalidation first, and then have a subsequent commit to relax it. I find
> it hard to believe that the behaviour in mainline at the moment is deliberate.
>
> That is, I'm not against optimising this, but I'd rather get it "obviously
> correct" first and the current code is definitely not that.
I wouldn't mind having this patch and patch 4 if the invalidation they
restore were in a correct state -- b3a81d0841a9 ("mm: fix KSM data
corruption") isn't correct to start with.
It is complicated, so please bear with me. Let's study this by looking
at examples this time.
> > > 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.
>
> Sorry, I don't follow what you're getting at here (page vs pte). Please can
> you point me to the code you're referring to? The case I'm worried about is
> code that holds sufficient locks (e.g. mmap_sem + ptl) finding an entry
> where !pte_write() and assuming (despite pte_dirty()) that there can't be
> any concurrent modifications to the mapped page. Granted, I haven't found
> anything doing that, but I could not convince myself that it would be a bug
> to write such code, either.
Example 1: memory corruption is still possible with patch 4 & 6
CPU0 CPU1 CPU2 CPU3
---- ---- ---- ----
userspace page writeback
[cache writable
PTE in TLB]
inc_tlb_flush_pending()
clean_record_pte()
pte_mkclean()
tlb_gather_mmu()
[set mm_tlb_flush_pending()]
clear_refs_write()
pte_wrprotect()
page_mkclean_one()
!pte_dirty() && !pte_write()
[true, no flush]
write page to disk
Write to page
[using stale PTE]
drop clean page
[data integrity compromised]
flush_tlb_range()
tlb_finish_mmu()
[flush (with patch 4)]
Example 2: why no flush when write-protecting is not a problem (after
we fix the problem correctly by adding mm_tlb_flush_pending()).
Case a:
CPU0 CPU1 CPU2 CPU3
---- ---- ---- ----
userspace page writeback
[cache writable
PTE in TLB]
inc_tlb_flush_pending()
clean_record_pte()
pte_mkclean()
clear_refs_write()
pte_wrprotect()
page_mkclean_one()
!pte_dirty() && !pte_write() &&
!mm_tlb_flush_pending()
[false: flush]
write page to disk
Write to page
[page fault]
drop clean page
[data integrity guaranteed]
flush_tlb_range()
Case b:
CPU0 CPU1 CPU2
---- ---- ----
userspace page writeback
[cache writable
PTE in TLB]
clear_refs_write()
pte_wrprotect()
[pte_dirty() is false]
page_mkclean_one()
!pte_dirty() && !pte_write() &&
!mm_tlb_flush_pending()
[true: no flush]
write page to disk
Write to page
[h/w tries to set
the dirty bit
but sees write-
protected PTE,
page fault]
drop clean page
[data integrity guaranteed]
Case c:
CPU0 CPU1 CPU2
---- ---- ----
userspace page writeback
[cache writable
PTE in TLB]
clear_refs_write()
pte_wrprotect()
[pte_dirty() is true]
page_mkclean_one()
!pte_dirty() && !pte_write() &&
!mm_tlb_flush_pending()
[false: flush]
write page to disk
Write to page
[page fault]
drop clean page
[data integrity guaranteed]
> > > 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
^^^^^^ shouldn't
Another typo, I apologize.
> > ("mm: fix KSM data corruption") in the first place.
>
> Sure, but if you check out b3a81d0841a9 then you have a fullmm TLB
> invalidation in tlb_finish_mmu(). 7a30df49f63a is what removed that, no?
>
> Will
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