linux-kernel - Re: [RFC v5 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

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Message-ID: <20180425144938.6mv7idgd2u2cyucu@kshutemo-mobl1>
Date:   Wed, 25 Apr 2018 17:49:38 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        mhocko@...nel.org, hch@...radead.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v5 PATCH] mm: shmem: make stat.st_blksize return huge page
 size if THP is on

On Wed, Apr 25, 2018 at 10:13:53PM +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@...ux.alibaba.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Suggested-by: Christoph Hellwig <hch@...radead.org>

Looks good to me:

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov