Re: [PATCH v2 14/14] arm64/mm: Add ptep_get_and_clear_full() to optimize process teardown

Barry Song
2023-11-30T05:58:06Z

On Thu, Nov 30, 2023 at 1:08 PM Alistair Popple <apopple@nvidia.com> wrote:
>
>
> Ryan Roberts <ryan.roberts@arm.com> writes:
>
> >>>> So if we do need to deal with racing HW, I'm pretty sure my v1 implementation is
> >>>> buggy because it iterated through the PTEs, getting and accumulating. Then
> >>>> iterated again, writing that final set of bits to all the PTEs. And the HW could
> >>>> have modified the bits during those loops. I think it would be possible to fix
> >>>> the race, but intuition says it would be expensive.
> >>>
> >>> So the issue as I understand it is subsequent iterations would see a
> >>> clean PTE after the first iteration returned a dirty PTE. In
> >>> ptep_get_and_clear_full() why couldn't you just copy the dirty/accessed
> >>> bit (if set) from the PTE being cleared to an adjacent PTE rather than
> >>> all the PTEs?
> >>
> >> The raciness I'm describing is the race between reading access/dirty from one
> >> pte and applying it to another. But yes I like your suggestion. if we do:
> >>
> >> pte = __ptep_get_and_clear_full(ptep)
> >>
> >> on the target pte, then we have grabbed access/dirty from it in a race-free
> >> manner. we can then loop from current pte up towards the top of the block until
> >> we find a valid entry (and I guess wrap at the top to make us robust against
> >> future callers clearing an an arbitrary order). Then atomically accumulate the
> >> access/dirty bits we have just saved into that new entry. I guess that's just a
> >> cmpxchg loop - there are already examples of how to do that correctly when
> >> racing the TLB.
> >>
> >> For most entries, we will just be copying up to the next pte. For the last pte,
> >> we would end up reading all ptes and determine we are the last one.
> >>
> >> What do you think?
> >
> > OK here is an attempt at something which solves the fragility. I think this is
> > now robust and will always return the correct access/dirty state from
> > ptep_get_and_clear_full() and ptep_get().
> >
> > But I'm not sure about performance; each call to ptep_get_and_clear_full() for
> > each pte in a contpte block will cause a ptep_get() to gather the access/dirty
> > bits from across the contpte block - which requires reading each pte in the
> > contpte block. So its O(n^2) in that sense. I'll benchmark it and report back.
> >
> > Was this the type of thing you were thinking of, Alistair?
>
> Yes, that is along the lines of what I was thinking. However I have
> added a couple of comments inline.
>
> > --8<--
> > arch/arm64/include/asm/pgtable.h | 23 ++++++++-
> > arch/arm64/mm/contpte.c | 81 ++++++++++++++++++++++++++++++++
> > arch/arm64/mm/fault.c | 38 +++++++++------
> > 3 files changed, 125 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 9bd2f57a9e11..6c295d277784 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -851,6 +851,7 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> > return pte_pmd(pte_modify(pmd_pte(pmd), newprot));
> > }
> >
> > +extern int __ptep_set_access_flags_notlbi(pte_t *ptep, pte_t entry);
> > extern int __ptep_set_access_flags(struct vm_area_struct *vma,
> > unsigned long address, pte_t *ptep,
> > pte_t entry, int dirty);
> > @@ -1145,6 +1146,8 @@ extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
> > extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
> > extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> > pte_t *ptep, pte_t pte, unsigned int nr);
> > +extern pte_t contpte_ptep_get_and_clear_full(struct mm_struct *mm,
> > + unsigned long addr, pte_t *ptep);
> > extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> > unsigned long addr, pte_t *ptep);
> > extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> > @@ -1270,12 +1273,28 @@ static inline void pte_clear(struct mm_struct *mm,
> > __pte_clear(mm, addr, ptep);
> > }
> >
> > +#define __HAVE_ARCH_PTEP_GET_AND_CLEAR_FULL
> > +static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
> > + unsigned long addr, pte_t *ptep, int full)
> > +{
> > + pte_t orig_pte = __ptep_get(ptep);
> > +
> > + if (!pte_valid_cont(orig_pte))
> > + return __ptep_get_and_clear(mm, addr, ptep);
> > +
> > + if (!full) {
> > + contpte_try_unfold(mm, addr, ptep, orig_pte);
> > + return __ptep_get_and_clear(mm, addr, ptep);
> > + }
> > +
> > + return contpte_ptep_get_and_clear_full(mm, addr, ptep);
> > +}
> > +
> > #define __HAVE_ARCH_PTEP_GET_AND_CLEAR
> > static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> > unsigned long addr, pte_t *ptep)
> > {
> > - contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> > - return __ptep_get_and_clear(mm, addr, ptep);
> > + return ptep_get_and_clear_full(mm, addr, ptep, 0);
> > }
> >
> > #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
> > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > index 2a57df16bf58..99b211118d93 100644
> > --- a/arch/arm64/mm/contpte.c
> > +++ b/arch/arm64/mm/contpte.c
> > @@ -145,6 +145,14 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> > for (i = 0; i < CONT_PTES; i++, ptep++) {
> > pte = __ptep_get(ptep);
> >
> > + /*
> > + * Deal with the partial contpte_ptep_get_and_clear_full() case,
> > + * where some of the ptes in the range may be cleared but others
> > + * are still to do. See contpte_ptep_get_and_clear_full().
> > + */
> > + if (!pte_valid(pte))
> > + continue;
> > +
> > if (pte_dirty(pte))
> > orig_pte = pte_mkdirty(orig_pte);
> >
> > @@ -257,6 +265,79 @@ void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> > }
> > EXPORT_SYMBOL(contpte_set_ptes);
> >
> > +pte_t contpte_ptep_get_and_clear_full(struct mm_struct *mm,
> > + unsigned long addr, pte_t *ptep)
> > +{
> > + /*
> > + * When doing a full address space teardown, we can avoid unfolding the
> > + * contiguous range, and therefore avoid the associated tlbi. Instead,
> > + * just get and clear the pte. The caller is promising to call us for
> > + * every pte, so every pte in the range will be cleared by the time the
> > + * final tlbi is issued.
> > + *
> > + * This approach requires some complex hoop jumping though, as for the
> > + * duration between returning from the first call to
> > + * ptep_get_and_clear_full() and making the final call, the contpte
> > + * block is in an intermediate state, where some ptes are cleared and
> > + * others are still set with the PTE_CONT bit. If any other APIs are
> > + * called for the ptes in the contpte block during that time, we have to
> > + * be very careful. The core code currently interleaves calls to
> > + * ptep_get_and_clear_full() with ptep_get() and so ptep_get() must be
> > + * careful to ignore the cleared entries when accumulating the access
> > + * and dirty bits - the same goes for ptep_get_lockless(). The only
> > + * other calls we might resonably expect are to set markers in the
> > + * previously cleared ptes. (We shouldn't see valid entries being set
> > + * until after the tlbi, at which point we are no longer in the
> > + * intermediate state). Since markers are not valid, this is safe;
> > + * set_ptes() will see the old, invalid entry and will not attempt to
> > + * unfold. And the new pte is also invalid so it won't attempt to fold.
> > + * We shouldn't see pte markers being set for the 'full' case anyway
> > + * since the address space is being torn down.
> > + *
> > + * The last remaining issue is returning the access/dirty bits. That
> > + * info could be present in any of the ptes in the contpte block.
> > + * ptep_get() will gather those bits from across the contpte block (for
> > + * the remaining valid entries). So below, if the pte we are clearing
> > + * has dirty or young set, we need to stash it into a pte that we are
> > + * yet to clear. This allows future calls to return the correct state
> > + * even when the info was stored in a different pte. Since the core-mm
> > + * calls from low to high address, we prefer to stash in the last pte of
> > + * the contpte block - this means we are not "dragging" the bits up
> > + * through all ptes and increases the chances that we can exit early
> > + * because a given pte will have neither dirty or young set.
> > + */
> > +
> > + pte_t orig_pte = __ptep_get_and_clear(mm, addr, ptep);
> > + bool dirty = pte_dirty(orig_pte);
> > + bool young = pte_young(orig_pte);
> > + pte_t *start;
> > +
> > + if (!dirty && !young)
> > + return contpte_ptep_get(ptep, orig_pte);
>
> I don't think we need to do this. If the PTE is !dirty && !young we can
> just return it. As you say we have to assume HW can set those flags at
> any time anyway so it doesn't get us much. This means in the common case
> we should only run through the loop setting the dirty/young flags once
> which should alay the performance concerns.
>
> However I am now wondering if we're doing the wrong thing trying to hide
> this down in the arch layer anyway. Perhaps it would be better to deal
> with this in the core-mm code after all.
>
> So how about having ptep_get_and_clear_full() clearing the PTEs for the
> entire cont block? We know by definition all PTEs should be pointing to

I truly believe we should clear all PTEs for the entire folio block. However,
if the existing api ptep_get_and_clear_full() is always handling a single one
PTE, we might keep its behaviour as is. On the other hand, clearing the
whole block isn't only required in fullmm case, it is also a requirement for
normal zap_pte_range() cases coming from madvise(DONTNEED) etc.

I do think we need a folio-level variant. as we are now supporting
pte-level large
folios, we need some new api to handle folio-level PTEs entirely as we always
have the needs to drop the whole folio rather than one by one when they are
compound.

> the same folio anyway, and it seems at least zap_pte_range() would cope
> with this just fine because subsequent iterations would just see
> pte_none() and continue the loop. I haven't checked the other call sites
> though, but in principal I don't see why we couldn't define
> ptep_get_and_clear_full() as being something that clears all PTEs
> mapping a given folio (although it might need renaming).
>
> This does assume you don't need to partially unmap a page in
> zap_pte_range (ie. end >= folio), but we're already making that
> assumption.
>
> > +
> > + start = contpte_align_down(ptep);
> > + ptep = start + CONT_PTES - 1;
> > +
> > + for (; ptep >= start; ptep--) {
> > + pte_t pte = __ptep_get(ptep);
> > +
> > + if (!pte_valid(pte))
> > + continue;
> > +
> > + if (dirty)
> > + pte = pte_mkdirty(pte);
> > +
> > + if (young)
> > + pte = pte_mkyoung(pte);
> > +
> > + __ptep_set_access_flags_notlbi(ptep, pte);
> > + return contpte_ptep_get(ptep, orig_pte);
> > + }
> > +
> > + return orig_pte;
> > +}
> > +EXPORT_SYMBOL(contpte_ptep_get_and_clear_full);
> > +
> > int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> > unsigned long addr, pte_t *ptep)
> > {
> > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > index d63f3a0a7251..b22216a8153c 100644
> > --- a/arch/arm64/mm/fault.c
> > +++ b/arch/arm64/mm/fault.c
> > @@ -199,19 +199,7 @@ static void show_pte(unsigned long addr)
> > pr_cont("\n");
> > }
> >
> > -/*
> > - * This function sets the access flags (dirty, accessed), as well as write
> > - * permission, and only to a more permissive setting.
> > - *
> > - * It needs to cope with hardware update of the accessed/dirty state by other
> > - * agents in the system and can safely skip the __sync_icache_dcache() call as,
> > - * like __set_ptes(), the PTE is never changed from no-exec to exec here.
> > - *
> > - * Returns whether or not the PTE actually changed.
> > - */
> > -int __ptep_set_access_flags(struct vm_area_struct *vma,
> > - unsigned long address, pte_t *ptep,
> > - pte_t entry, int dirty)
> > +int __ptep_set_access_flags_notlbi(pte_t *ptep, pte_t entry)
> > {
> > pteval_t old_pteval, pteval;
> > pte_t pte = __ptep_get(ptep);
> > @@ -238,10 +226,30 @@ int __ptep_set_access_flags(struct vm_area_struct *vma,
> > pteval = cmpxchg_relaxed(&pte_val(*ptep), old_pteval, pteval);
> > } while (pteval != old_pteval);
> >
> > + return 1;
> > +}
> > +
> > +/*
> > + * This function sets the access flags (dirty, accessed), as well as write
> > + * permission, and only to a more permissive setting.
> > + *
> > + * It needs to cope with hardware update of the accessed/dirty state by other
> > + * agents in the system and can safely skip the __sync_icache_dcache() call as,
> > + * like __set_ptes(), the PTE is never changed from no-exec to exec here.
> > + *
> > + * Returns whether or not the PTE actually changed.
> > + */
> > +int __ptep_set_access_flags(struct vm_area_struct *vma,
> > + unsigned long address, pte_t *ptep,
> > + pte_t entry, int dirty)
> > +{
> > + int changed = __ptep_set_access_flags_notlbi(ptep, entry);
> > +
> > /* Invalidate a stale read-only entry */
> > - if (dirty)
> > + if (changed && dirty)
> > flush_tlb_page(vma, address);
> > - return 1;
> > +
> > + return changed;
> > }
> >
> > static bool is_el1_instruction_abort(unsigned long esr)
> > --8<--
>

Thanks
Barry