Re: [Patch v2] mm/rmap.c: split huge pmd when it really is

[Matthew Wilcox <willy@infradead.org>]

On Tue, Dec 24, 2019 at 09:56:02AM +0800, Wei Yang wrote:
> On Mon, Dec 23, 2019 at 03:11:20PM -0800, Matthew Wilcox wrote:
> >On Tue, Dec 24, 2019 at 06:28:56AM +0800, Wei Yang wrote:
> >> When page is not NULL, function is called by try_to_unmap_one() with
> >> TTU_SPLIT_HUGE_PMD set. There are two cases to call try_to_unmap_one()
> >> with TTU_SPLIT_HUGE_PMD set:
> >> 
> >>   * unmap_page()
> >>   * shrink_page_list()
> >> 
> >> In both case, the page passed to try_to_unmap_one() is PageHead() of the
> >> THP. If this page's mapping address in process is not HPAGE_PMD_SIZE
> >> aligned, this means the THP is not mapped as PMD THP in this process.
> >> This could happen when we do mremap() a PMD size range to an un-aligned
> >> address.
> >> 
> >> Currently, this case is handled by following check in __split_huge_pmd()
> >> luckily.
> >> 
> >>   page != pmd_page(*pmd)
> >> 
> >> This patch checks the address to skip some work.
> >
> >The description here is confusing to me.
> >
> 
> Sorry for the confusion.
> 
> Below is my understanding, if not correct or proper, just let me know :-)
> 
> According to current comment in __split_huge_pmd(), we check pmd_page with
> page for migration case. While actually, this check also helps in the
> following two cases when page already split-ed:
> 
>    * page just split-ed in place
>    * page split-ed and moved to non-PMD aligned address
> 
> In both cases, pmd_page() is pointing to the PTE level page table. That's why
> we don't split one already split-ed THP page.
> 
> If current code really intend to cover these two cases, sorry for my poor
> understanding.
> 
> >> +	/*
> >> +	 * When page is not NULL, function is called by try_to_unmap_one()
> >> +	 * with TTU_SPLIT_HUGE_PMD set. There are two places set
> >> +	 * TTU_SPLIT_HUGE_PMD
> >> +	 *
> >> +	 *     unmap_page()
> >> +	 *     shrink_page_list()
> >> +	 *
> >> +	 * In both cases, the "page" here is the PageHead() of a THP.
> >> +	 *
> >> +	 * If the page is not a PMD mapped huge page, e.g. after mremap(), it
> >> +	 * is not necessary to split it.
> >> +	 */
> >> +	if (page && !IS_ALIGNED(address, HPAGE_PMD_SIZE))
> >> +		return;
> >
> >Repeating 75% of it as comments doesn't make it any less confusing.  And
> >it feels like we're digging a pothole for someone to fall into later.
> >Why not make it make sense ...
> >
> >	if (page && !IS_ALIGNED(address, page_size(page))
> >		return;
> 
> Hmm... Use HPAGE_PMD_SIZE here wants to emphasize we want the address to be
> PMD aligned. If just use page_size() here, may confuse the audience?

I'm OK with using HPAGE_PMD_SIZE here.  I was trying to future-proof
this function for supporting 64kB pages with a 4kB page size on ARM,
but this function will need changes for that anyway, so I'm OK with
your suggestion.