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SubjectRe: [RFC PATCH] fs: introduce ST_HUGE flag and set it to tmpfs and hugetlbfs
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On 4/17/18 4:22 PM, Matthew Wilcox wrote:
> On Wed, Apr 18, 2018 at 05:08:13AM +0800, Yang Shi wrote:
>> 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.
> Hm. What's the plan for communicating support for page sizes other
> than PMD page sizes? I know ARM has several different page sizes,
> as do PA-RISC and ia64. Even x86 might support 1G page sizes through
> tmpfs one day.

For THP page size, we already have
/sys/kernel/mm/transparent_hugepage/hpage_pmd_size exported. The
applications could read this to get the THP size. If PUD size THP
supported is added later, we can export hpage_pud_size.

Please see the below commit log for more details:

commit 49920d28781dcced10cd30cb9a938e7d045a1c94
Author: Hugh Dickins <hughd@google.com>
Date:   Mon Dec 12 16:44:50 2016 -0800

    mm: make transparent hugepage size public

    Test programs want to know the size of a transparent hugepage. While it
    is commonly the same as the size of a hugetlbfs page (shown as
    Hugepagesize in /proc/meminfo), that is not always so: powerpc
    implements transparent hugepages in a different way from hugetlbfs
    pages, so it's coincidence when their sizes are the same; and x86 and
    others can support more than one hugetlbfs page size.

    Add /sys/kernel/mm/transparent_hugepage/hpage_pmd_size to show the THP
    size in bytes - it's the same for Anonymous and Shmem hugepages.  Call
    it hpage_pmd_size (after HPAGE_PMD_SIZE) rather than hpage_size, in
case
    some transparent support for pud and pgd pages is added later.


Thanks,
Yang


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