Re: [PATCH v7 5/6] mm,thp: add read-only THP support for (non-shmem) FS

[Hillf Danton <hdanton@sina.com>]

Hello

On Sun, 23 Jun 2019 13:48:47 +0800 Song Liu wrote:
> This patch is (hopefully) the first step to enable THP for non-shmem
> filesystems.
> 
> This patch enables an application to put part of its text sections to THP
> via madvise, for example:
> 
>     madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE);
> 
> We tried to reuse the logic for THP on tmpfs.
> 
> Currently, write is not supported for non-shmem THP. khugepaged will only
> process vma with VM_DENYWRITE. The next patch will handle writes, which
> would only happen when the vma with VM_DENYWRITE is unmapped.
> 
> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
> feature.
> 
> Acked-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
>  mm/Kconfig      | 11 ++++++
>  mm/filemap.c    |  4 +--
>  mm/khugepaged.c | 90 ++++++++++++++++++++++++++++++++++++++++---------
>  mm/rmap.c       | 12 ++++---
>  4 files changed, 96 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index f0c76ba47695..0a8fd589406d 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -762,6 +762,17 @@ config GUP_BENCHMARK
> 
>  	  See tools/testing/selftests/vm/gup_benchmark.c
> 
> +config READ_ONLY_THP_FOR_FS
> +	bool "Read-only THP for filesystems (EXPERIMENTAL)"
> +	depends on TRANSPARENT_HUGE_PAGECACHE && SHMEM
> +
The ext4 mentioned in the cover letter, along with the subject line of
this patch, suggests the scissoring of SHMEM.

> +	help
> +	  Allow khugepaged to put read-only file-backed pages in THP.
> +
> +	  This is marked experimental because it is a new feature. Write
> +	  support of file THPs will be developed in the next few release
> +	  cycles.
> +
>  config ARCH_HAS_PTE_SPECIAL
>  	bool

Hillf