Re: [PATCH v6 5/6] khugepaged: enable collapse pmd for pte-mapped THP

[Song Liu <songliubraving@fb.com>]

> On Jun 22, 2019, at 10:48 PM, Song Liu <songliubraving@fb.com> wrote:
> 
> khugepaged needs exclusive mmap_sem to access page table. When it fails
> to lock mmap_sem, the page will fault in as pte-mapped THP. As the page
> is already a THP, khugepaged will not handle this pmd again.
> 
> This patch enables the khugepaged to retry retract_page_tables().
> 
> A new flag AS_COLLAPSE_PMD is introduced to show the address_space may
> contain pte-mapped THPs. When khugepaged fails to trylock the mmap_sem,
> it sets AS_COLLAPSE_PMD. Then, at a later time, khugepaged will retry
> compound pages in this address_space.
> 
> Since collapse may happen at an later time, some pages may already fault
> in. To handle these pages properly, it is necessary to prepare the pmd
> before collapsing. prepare_pmd_for_collapse() is introduced to prepare
> the pmd by removing rmap, adjusting refcount and mm_counter.
> 
> prepare_pmd_for_collapse() also double checks whether all ptes in this
> pmd are mapping to the same THP. This is necessary because some subpage
> of the THP may be replaced, for example by uprobe. In such cases, it
> is not possible to collapse the pmd, so we fall back.
> 
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
> include/linux/pagemap.h |  1 +
> mm/khugepaged.c         | 69 +++++++++++++++++++++++++++++++++++------
> 2 files changed, 60 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 9ec3544baee2..eac881de2a46 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -29,6 +29,7 @@ enum mapping_flags {
> 	AS_EXITING	= 4, 	/* final truncate in progress */
> 	/* writeback related tags are not used */
> 	AS_NO_WRITEBACK_TAGS = 5,
> +	AS_COLLAPSE_PMD = 6,	/* try collapse pmd for THP */
> };
> 
> /**
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index a4f90a1b06f5..9b980327fd9b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1254,7 +1254,47 @@ static void collect_mm_slot(struct mm_slot *mm_slot)
> }
> 
> #if defined(CONFIG_SHMEM) && defined(CONFIG_TRANSPARENT_HUGE_PAGECACHE)
> -static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
> +
> +/* return whether the pmd is ready for collapse */
> +bool prepare_pmd_for_collapse(struct vm_area_struct *vma, pgoff_t pgoff,
> +			      struct page *hpage, pmd_t *pmd)


kbuild test robot reported I missed "static" here. But I am holding off a 
newer version that just fixes this. 

Thanks, 
Song