Re: [RFC v4 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

[Yang Shi <yang.shi@linux.alibaba.com>]

On 4/24/18 5:23 AM, Kirill A. Shutemov wrote:
> On Tue, Apr 24, 2018 at 12:00:50PM +0800, Yang Shi wrote:
>> Since tmpfs THP was supported in 4.8, hugetlbfs is not the only
>> filesystem with huge page support anymore. tmpfs can use huge page via
>> THP when mounting by "huge=" mount option.
>>
>> When applications use huge page on hugetlbfs, it just need check the
>> filesystem magic number, but it is not enough for tmpfs. Make
>> stat.st_blksize return huge page size if it is mounted by appropriate
>> "huge=" option to give applications a hint to optimize the behavior with
>> THP.
>>
>> Some applications may not do wisely with THP. For example, QEMU may mmap
>> file on non huge page aligned hint address with MAP_FIXED, which results
>> in no pages are PMD mapped even though THP is used. Some applications
>> may mmap file with non huge page aligned offset. Both behaviors make THP
>> pointless.
>>
>> statfs.f_bsize still returns 4KB for tmpfs since THP could be split, and it
>> also may fallback to 4KB page silently if there is not enough huge page.
>> Furthermore, different f_bsize makes max_blocks and free_blocks
>> calculation harder but without too much benefit. Returning huge page
>> size via stat.st_blksize sounds good enough.
>>
>> Since PUD size huge page for THP has not been supported, now it just
>> returns HPAGE_PMD_SIZE.
>>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Suggested-by: Christoph Hellwig <hch@infradead.org>
>> ---
>> v3 --> v4:
>> * Rework the commit log per the education from Michal and Kirill
>> * Fix build error if CONFIG_TRANSPARENT_HUGEPAGE is disabled
>> v2 --> v3:
>> * Use shmem_sb_info.huge instead of global variable per Michal's comment
>> v2 --> v1:
>> * Adopted the suggestion from hch to return huge page size via st_blksize
>>    instead of creating a new flag.
>>
>>   mm/shmem.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index b859192..19b8055 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -988,6 +988,7 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
>>   {
>>   	struct inode *inode = path->dentry->d_inode;
>>   	struct shmem_inode_info *info = SHMEM_I(inode);
>> +	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>>   
>>   	if (info->alloced - info->swapped != inode->i_mapping->nrpages) {
>>   		spin_lock_irq(&info->lock);
>> @@ -995,6 +996,11 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
>>   		spin_unlock_irq(&info->lock);
>>   	}
>>   	generic_fillattr(inode, stat);
>> +#ifdef CONFIG_TRANSPARENT_HUGE_PAGECACHE
>> +	if (sbinfo->huge > 0)
> No ifdeffery, please.
>
> And we probably want to check if shmem_huge is 'force'.
>
> Something like this?
>
> 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
> 		 (shmem_huge == SHMEM_HUGE_FORCE || sbinfo->huge))

BTW, it sounds we also need check shmem_huge != SHMEM_HUGE_DENY, right? 
Otherwise it still return huge page size, but in fact no huge page can 
be allocated with 'deny'.

So, how about:

if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
          (shmem_huge == SHMEM_HUGE_FORCE || sbinfo->huge) &&
          shmem_huge != SHMEM_HUGE_DENY)

Thanks,
Yang

>
>> +		stat->blksize = HPAGE_PMD_SIZE;
>> +#endif
>> +	
>>   	return 0;
>>   }
>>   
>> -- 
>> 1.8.3.1
>>