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From:   Song Liu <songliubraving@fb.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
CC:     Linux-MM <linux-mm@kvack.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
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        "william.kucharski@oracle.com" <william.kucharski@oracle.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>
Subject: Re: [PATCH v5 6/6] mm,thp: avoid writes to file with THP in pagecache
Thread-Topic: [PATCH v5 6/6] mm,thp: avoid writes to file with THP in
 pagecache
Thread-Index: AQHVJ6piUxjnrZaN9U27pEvDQgjQg6amFUcAgAAA5oA=
Date:   Fri, 21 Jun 2019 13:10:54 +0000
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> On Jun 21, 2019, at 6:07 AM, Kirill A. Shutemov <kirill@shutemov.name> wr=
ote:
>=20
> On Thu, Jun 20, 2019 at 01:53:48PM -0700, Song Liu wrote:
>> In previous patch, an application could put part of its text section in
>> THP via madvise(). These THPs will be protected from writes when the
>> application is still running (TXTBSY). However, after the application
>> exits, the file is available for writes.
>>=20
>> This patch avoids writes to file THP by dropping page cache for the file
>> when the file is open for write. A new counter nr_thps is added to struc=
t
>> address_space. In do_last(), if the file is open for write and nr_thps
>> is non-zero, we drop page cache for the whole file.
>>=20
>> Signed-off-by: Song Liu <songliubraving@fb.com>
>> ---
>> fs/inode.c         |  3 +++
>> fs/namei.c         | 22 +++++++++++++++++++++-
>> include/linux/fs.h | 31 +++++++++++++++++++++++++++++++
>> mm/filemap.c       |  1 +
>> mm/khugepaged.c    |  4 +++-
>> 5 files changed, 59 insertions(+), 2 deletions(-)
>>=20
>> diff --git a/fs/inode.c b/fs/inode.c
>> index df6542ec3b88..518113a4e219 100644
>> --- a/fs/inode.c
>> +++ b/fs/inode.c
>> @@ -181,6 +181,9 @@ int inode_init_always(struct super_block *sb, struct=
 inode *inode)
>> 	mapping->flags =3D 0;
>> 	mapping->wb_err =3D 0;
>> 	atomic_set(&mapping->i_mmap_writable, 0);
>> +#ifdef CONFIG_READ_ONLY_THP_FOR_FS
>> +	atomic_set(&mapping->nr_thps, 0);
>> +#endif
>> 	mapping_set_gfp_mask(mapping, GFP_HIGHUSER_MOVABLE);
>> 	mapping->private_data =3D NULL;
>> 	mapping->writeback_index =3D 0;
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 20831c2fbb34..de64f24b58e9 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -3249,6 +3249,22 @@ static int lookup_open(struct nameidata *nd, stru=
ct path *path,
>> 	return error;
>> }
>>=20
>> +/*
>> + * The file is open for write, so it is not mmapped with VM_DENYWRITE. =
If
>> + * it still has THP in page cache, drop the whole file from pagecache
>> + * before processing writes. This helps us avoid handling write back of
>> + * THP for now.
>> + */
>> +static inline void release_file_thp(struct file *file)
>> +{
>> +#ifdef CONFIG_READ_ONLY_THP_FOR_FS
>> +	struct inode *inode =3D file_inode(file);
>> +
>> +	if (inode_is_open_for_write(inode) && filemap_nr_thps(inode->i_mapping=
))
>> +		truncate_pagecache(inode, 0);
>> +#endif
>> +}
>> +
>> /*
>>  * Handle the last step of open()
>>  */
>> @@ -3418,7 +3434,11 @@ static int do_last(struct nameidata *nd,
>> 		goto out;
>> opened:
>> 	error =3D ima_file_check(file, op->acc_mode);
>> -	if (!error && will_truncate)
>> +	if (error)
>> +		goto out;
>> +
>> +	release_file_thp(file);
>=20
> What protects against re-fill the file with THP in parallel?

khugepaged would only process vma with VM_DENYWRITE. So once the
file is open for write (i_write_count > 0), khugepage will not=20
collapse the pages.=20

Thanks,
Song