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From:   Song Liu <songliubraving@fb.com>
To:     "Huang, Kai" <kai.huang@intel.com>
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Subject: Re: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem)
 FS
Thread-Topic: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem)
 FS
Thread-Index: AQHVKurnm0pZpZevHki8fTgvnJjHiqbZD5wAgAa5fgA=
Date:   Sun, 28 Jul 2019 06:41:23 +0000
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> On Jul 23, 2019, at 4:59 PM, Huang, Kai <kai.huang@intel.com> wrote:
>=20
> 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.
>>=20
>> This patch enables an application to put part of its text sections to TH=
P
>> via madvise, for example:
>>=20
>>    madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE);
>>=20
>> We tried to reuse the logic for THP on tmpfs.
>>=20
>> Currently, write is not supported for non-shmem THP. khugepaged will onl=
y
>> 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.
>>=20
>> The next patch will handle writes, which would only happen when the all
>> the vmas with VM_DENYWRITE are unmapped.
>>=20
>> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
>> feature.
>>=20
>> 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 | 94 +++++++++++++++++++++++++++++++++++++++++--------
>> mm/rmap.c       | 12 ++++---
>> 4 files changed, 100 insertions(+), 21 deletions(-)
>>=20
>> 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
>>=20
>> 	  See tools/testing/selftests/vm/gup_benchmark.c
>>=20
>> +config READ_ONLY_THP_FOR_FS
>> +	bool "Read-only THP for filesystems (EXPERIMENTAL)"
>> +	depends on TRANSPARENT_HUGE_PAGECACHE && SHMEM
>=20
> Hi,
>=20
> 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=
=20
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=20
dependency at this stage.

Thanks,
Song