Re: [PATCH v2 2/2] mm: remove redundant smp_wmb()

[Kirill A. Shutemov]

On Tue, Aug 31, 2021 at 09:21:11PM +0800, 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 pmd_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 pmd_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>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov