Re: [RFC PATCH] fs: introduce ST_HUGE flag and set it to tmpfs and hugetlbfs

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

On 4/18/18 3:27 AM, Christoph Hellwig wrote:
> On Wed, Apr 18, 2018 at 05:08:13AM +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. So, introduce
>> ST_HUGE flag to statfs if super block has SB_HUGE set which indicates
>> huge page is supported on the specific filesystem.
>>
>> Some applications could benefit from this change, for example QEMU.
>> When use mmap file as guest VM backend memory, QEMU typically mmap the
>> file size plus one extra page. If the file is on hugetlbfs the extra
>> page is huge page size (i.e. 2MB), but it is still 4KB on tmpfs even
>> though THP is enabled. tmpfs THP requires VMA is huge page aligned, so
>> if 4KB page is used THP will not be used at all. The below /proc/meminfo
>> fragment shows the THP use of QEMU with 4K page:
>>
>> ShmemHugePages:   679936 kB
>> ShmemPmdMapped:        0 kB
>>
>> With ST_HUGE flag, QEMU can get huge page, then /proc/meminfo looks
>> like:
>>
>> ShmemHugePages:    77824 kB
>> ShmemPmdMapped:     6144 kB
>>
>> With this flag, the applications can know if huge page is supported on
>> the filesystem then optimize the behavior of the applications
>> accordingly. Although the similar function can be implemented in
>> applications by traversing the mount options, it looks more convenient
>> if kernel can provide such flag.
>>
>> Even though ST_HUGE is set, f_bsize still returns 4KB for tmpfs since
>> THP could be split, and it also my fallback to 4KB page silently if
>> there is not enough huge page.
> Seems like your should report it through the st_blksize field of struct
> stat then, instead of introducing a not very useful binary field then.

Yes, thanks for the suggestion. I did think about it before I went with 
the new flag. Not like hugetlb, THP will *not* guarantee huge page is 
used all the time, it may fallback to regular 4K page or may get split. 
I'm not sure how the applications use f_bsize field, it might break 
existing applications and the value might be abused by applications to 
have counter optimization. So, IMHO, a new flag may sound safer.

Yang