linux-kernel - Re: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem) FS

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Date:   Sun, 28 Jul 2019 06:41:23 +0000
From:   Song Liu <songliubraving@...com>
To:     "Huang, Kai" <kai.huang@...el.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "hdanton@...a.com" <hdanton@...a.com>,
        "Kernel Team" <Kernel-team@...com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>
Subject: Re: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem)
 FS



> On Jul 23, 2019, at 4:59 PM, Huang, Kai <kai.huang@...el.com> wrote:
> 
> On Mon, 2019-06-24 at 17:12 -0700, 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. sys_mmap() ignores VM_DENYWRITE requests
>> (see ksys_mmap_pgoff). The only way to create vma with VM_DENYWRITE is
>> execve(). This requirement limits non-shmem THP to text sections.
>> 
>> The next patch will handle writes, which would only happen when the all
>> the vmas with VM_DENYWRITE are unmapped.
>> 
>> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
>> feature.
>> 
>> Acked-by: Rik van Riel <riel@...riel.com>
>> Signed-off-by: Song Liu <songliubraving@...com>
>> ---
>> mm/Kconfig      | 11 ++++++
>> mm/filemap.c    |  4 +--
>> mm/khugepaged.c | 94 +++++++++++++++++++++++++++++++++++++++++--------
>> mm/rmap.c       | 12 ++++---
>> 4 files changed, 100 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
> 
> Hi,
> 
> Maybe a stupid question since I am new, but why does it depend on SHMEM?

Not stupid at all. :)

We reuse a lot of code for shmem thp, thus the dependency. Technically, we 
can remove the dependency. However, we will remove this config option when
THP for FS is more mature. So it doesn't make sense to resolve the 
dependency at this stage.

Thanks,
Song