Re: [PATCH v5 6/6] mm,thp: avoid writes to file with THP in pagecache

[Song Liu]

> On Jun 21, 2019, at 6:07 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> 
> 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.
>> 
>> 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 struct
>> 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.
>> 
>> Signed-off-by: Song Liu <songliubraving@xxxxxx>
>> ---
>> 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(-)
>> 
>> 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 = 0;
>> 	mapping->wb_err = 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 = NULL;
>> 	mapping->writeback_index = 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, struct path *path,
>> 	return error;
>> }
>> 
>> +/*
>> + * 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 = 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 = ima_file_check(file, op->acc_mode);
>> -	if (!error && will_truncate)
>> +	if (error)
>> +		goto out;
>> +
>> +	release_file_thp(file);
> 
> 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 
collapse the pages. 

Thanks,
Song