lkml.org 
[lkml]   [2020]   [Jan]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [Patch v2] mm/rmap.c: split huge pmd when it really is
On Fri, Jan 03, 2020 at 09:05:54PM +0800, Wei Yang wrote:
> On Fri, Jan 03, 2020 at 03:18:46PM +0800, Wei Yang 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.
> >
> >I am sorry to forget address Kirill's comment in 1st version.
> >
> >The first one is the performance difference after this change for a PTE
> >mappged THP.
> >
> >Here is the result:(in cycle)
> >
> > Before Patched
> >
> > 963 195
> > 988 40
> > 895 78
> >
> >Average 948 104
> >
> >So the change reduced 90% time for function split_huge_pmd_address().

Right.

But do we have a scenario, where the performance of
split_huge_pmd_address() matters? I mean, it it called as part of rmap
walk, attempt to split huge PMD where we don't have huge PMD should be
within noise.

> >For the 2nd comment, the vma check. Let me take a further look to analysis.
> >
> >Thanks for Kirill's suggestion.
> >
>
> For 2nd comment, check vma could hold huge page.
>
> You mean do this check ?
>
> vma->vm_start <= address && vma->vm_end >= address + HPAGE_PMD_SIZE
>
> This happens after munmap a partial of the THP range? After doing so, we can
> skip split_pmd for this case.

Okay, you are right. This kind of check would not be safe as we
split_huge_pmd_address() after adjusting VMA with expectation of splitting
PMD on boundary of the VMA.

--
Kirill A. Shutemov

\
 
 \ /
  Last update: 2020-01-03 14:27    [W:0.066 / U:0.600 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site