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From: Yang Shi <yang.shi@linux.alibaba.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: viro@zeniv.linux.org.uk, nyc@holomorphy.com,
	mike.kravetz@oracle.com, kirill.shutemov@linux.intel.com,
	hughd@google.com, akpm@linux-foundation.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] fs: introduce ST_HUGE flag and set it to tmpfs and hugetlbfs
Date: Wed, 18 Apr 2018 11:18:25 -0700	[thread overview]
Message-ID: <73090d4b-6831-805b-8b9d-5dff267428d9@linux.alibaba.com> (raw)
In-Reply-To: <20180418102744.GA10397@infradead.org>



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

  reply	other threads:[~2018-04-18 18:18 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-17 21:08 Yang Shi
2018-04-17 21:31 ` Andrew Morton
2018-04-17 21:51   ` Yang Shi
2018-04-17 23:22 ` Matthew Wilcox
2018-04-17 23:37   ` Yang Shi
2018-04-18 10:27 ` Christoph Hellwig
2018-04-18 18:18   ` Yang Shi [this message]
2018-04-19  8:28     ` Christoph Hellwig
2018-04-19  9:05       ` Kirill A. Shutemov
2018-04-20  0:18       ` Yang Shi
2018-04-18 20:26 ` Mike Kravetz
2018-04-18 20:53   ` Yang Shi
2018-04-19  9:01   ` Kirill A. Shutemov

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