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

[Vlastimil Babka]

On 8/28/21 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(),

It's named pmd_install()?

> __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().

Same here.

> 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 */

So, could it? :)