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

On 4/17/18 2:31 PM, Andrew Morton wrote:

On Wed, 18 Apr 2018 05:08:13 +0800 Yang Shi [off-list ref] wrote:

quoted

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.

And, set the flag for hugetlbfs as well to keep the consistency, and the
applications don't have to know what filesystem is used to use huge
page, just need to check ST_HUGE flag.

Patch is simple enough, although I'm having trouble forming an opinion
about it ;)

It will call for an update to the statfs(2) manpage.  I'm not sure
which of linux-man@vger.kernel.org, mtk.manpages@gmail.com and
linux-api@vger.kernel.org is best for that, so I'd cc all three...

Thanks, Andrew. Added cc to those 3 lists.