Re: [PATCH v1 2/2] mm: remove redundant smp_wmb()

[Qi Zheng]

On 2021/8/31 PM6:02, David Hildenbrand wrote:
> On 28.08.21 06:23, Qi Zheng wrote:
> > The smp_wmb() which is in the __pte_alloc() is used to
> > ensure all ptes setup is visible before the pte is made
> > visible to other CPUs by being put into page tables. We
> > only need this when the pte is actually populated, so
> > move it to pte_install(). __pte_alloc_kernel(),
> > __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar
> > to this case.
> >
> > We can also defer smp_wmb() to the place where the pmd entry
> > is really populated by preallocated pte. There are two kinds
> > of user of preallocated pte, one is filemap & finish_fault(),
> > another is THP. The former does not need another smp_wmb()
> > because the smp_wmb() has been done by pte_install().
> > Fortunately, the latter also does not need another smp_wmb()
> > because there is already a smp_wmb() before populating the
> > new pte when the THP uses a preallocated pte to split a huge
> > pmd.
> >
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >   mm/memory.c         | 47 
> > ++++++++++++++++++++---------------------------
> >   mm/sparse-vmemmap.c |  2 +-
> >   2 files changed, 21 insertions(+), 28 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index ef7b1762e996..9c7534187454 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -439,6 +439,20 @@ void pmd_install(struct mm_struct *mm, pmd_t 
> > *pmd, pgtable_t *pte)
> >       if (likely(pmd_none(*pmd))) {    /* Has another populated it ? */
> >           mm_inc_nr_ptes(mm);
> > +        /*
> > +         * Ensure all pte setup (eg. pte page lock and page clearing) 
> > are
> > +         * visible before the pte is made visible to other CPUs by being
> > +         * put into page tables.
> > +         *
> > +         * The other side of the story is the pointer chasing in the 
> > page
> > +         * table walking code (when walking the page table without 
> > locking;
> > +         * ie. most of the time). Fortunately, these data accesses 
> > consist
> > +         * of a chain of data-dependent loads, meaning most CPUs (alpha
> > +         * being the notable exception) will already guarantee loads are
> > +         * seen in-order. See the alpha page table accessors for the
> > +         * smp_rmb() barriers in page table walking code.
> > +         */
> > +        smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
> >           pmd_populate(mm, pmd, *pte);
> >           *pte = NULL;
> >       }
> > @@ -451,21 +465,6 @@ int __pte_alloc(struct mm_struct *mm, pmd_t *pmd)
> >       if (!new)
> >           return -ENOMEM;
> > -    /*
> > -     * Ensure all pte setup (eg. pte page lock and page clearing) are
> > -     * visible before the pte is made visible to other CPUs by being
> > -     * put into page tables.
> > -     *
> > -     * The other side of the story is the pointer chasing in the page
> > -     * table walking code (when walking the page table without locking;
> > -     * ie. most of the time). Fortunately, these data accesses consist
> > -     * of a chain of data-dependent loads, meaning most CPUs (alpha
> > -     * being the notable exception) will already guarantee loads are
> > -     * seen in-order. See the alpha page table accessors for the
> > -     * smp_rmb() barriers in page table walking code.
> > -     */
> > -    smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */
> > -
> >       pmd_install(mm, pmd, &new);
> >       if (new)
> >           pte_free(mm, new);
> > @@ -478,10 +477,9 @@ int __pte_alloc_kernel(pmd_t *pmd)
> >       if (!new)
> >           return -ENOMEM;
> > -    smp_wmb(); /* See comment in __pte_alloc */
> > -
> >       spin_lock(&init_mm.page_table_lock);
> >       if (likely(pmd_none(*pmd))) {    /* Has another populated it ? */
> > +        smp_wmb(); /* See comment in pmd_install() */
> >           pmd_populate_kernel(&init_mm, pmd, new);
> >           new = NULL;
> >       }
> > @@ -3857,7 +3855,6 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
> >           vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
> >           if (!vmf->prealloc_pte)
> >               return VM_FAULT_OOM;
> > -        smp_wmb(); /* See comment in __pte_alloc() */
> >       }
> >       ret = vma->vm_ops->fault(vmf);
> > @@ -3919,7 +3916,6 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, 
> > struct page *page)
> >           vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
> >           if (!vmf->prealloc_pte)
> >               return VM_FAULT_OOM;
> > -        smp_wmb(); /* See comment in __pte_alloc() */
> >       }
> >       vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> > @@ -4144,7 +4140,6 @@ static vm_fault_t do_fault_around(struct 
> > vm_fault *vmf)
> >           vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm);
> >           if (!vmf->prealloc_pte)
> >               return VM_FAULT_OOM;
> > -        smp_wmb(); /* See comment in __pte_alloc() */
> >       }
> >       return vmf->vma->vm_ops->map_pages(vmf, start_pgoff, end_pgoff);
> > @@ -4819,13 +4814,13 @@ int __p4d_alloc(struct mm_struct *mm, pgd_t 
> > *pgd, unsigned long address)
> >       if (!new)
> >           return -ENOMEM;
> > -    smp_wmb(); /* See comment in __pte_alloc */
> > -
> >       spin_lock(&mm->page_table_lock);
> >       if (pgd_present(*pgd))        /* Another has populated it */
> >           p4d_free(mm, new);
> > -    else
> > +    else {
> > +        smp_wmb(); /* See comment in pmd_install() */
> >           pgd_populate(mm, pgd, new);
> > +    }
>
> Nit:
>
> if () {
>
> } else {
>
> }
>
> see Documentation/process/coding-style.rst
>
> "This does not apply if only one branch of a conditional statement is a 
> single statement; in the latter case use braces in both branches:"

Got it.

> Apart from that, I think this is fine,
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks,
Qi