Re: [PATCH v3 3/5] mm, thp: introduce FOLL_SPLIT_PMD

[Kirill A. Shutemov]

On Thu, Jun 13, 2019 at 03:03:01PM +0000, Song Liu wrote:
> 
> 
> > On Jun 13, 2019, at 7:16 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> > 
> > On Thu, Jun 13, 2019 at 01:57:30PM +0000, Song Liu wrote:
> >>> And I'm not convinced that it belongs here at all. User requested PMD
> >>> split and it is done after split_huge_pmd(). The rest can be handled by
> >>> the caller as needed.
> >> 
> >> I put this part here because split_huge_pmd() for file-backed THP is
> >> not really done after split_huge_pmd(). And I would like it done before
> >> calling follow_page_pte() below. Maybe we can still do them here, just 
> >> for file-backed THPs?
> >> 
> >> If we would move it, shall we move to callers of follow_page_mask()? 
> >> In that case, we will probably end up with similar code in two places:
> >> __get_user_pages() and follow_page(). 
> >> 
> >> Did I get this right?
> > 
> > Would it be enough to replace pte_offset_map_lock() in follow_page_pte()
> > with pte_alloc_map_lock()?
> 
> This is similar to my previous version:
> 
> +		} else {  /* flags & FOLL_SPLIT_PMD */
> +			pte_t *pte;
> +			spin_unlock(ptl);
> +			split_huge_pmd(vma, pmd, address);
> +			pte = get_locked_pte(mm, address, &ptl);
> +			if (!pte)
> +				return no_page_table(vma, flags);
> +			spin_unlock(ptl);
> +			ret = 0;
> +		}
> 
> I think this is cleaner than use pte_alloc_map_lock() in follow_page_pte(). 
> What's your thought on these two versions (^^^ vs. pte_alloc_map_lock)?

It's additional lock-unlock cycle and few more lines of code...

> > This will leave bunch not populated PTE entries, but it is fine: they will
> > be populated on the next access to them.
> 
> We need to handle page fault during next access, right? Since we already
> allocated everything, we can just populate the PTE entries and saves a
> lot of page faults (assuming we will access them later). 

Not a lot due to faultaround and they may never happen, but you need to
tear down the mapping any way.

-- 
 Kirill A. Shutemov