linux-kernel - Re: [RFC PATCH] mm: shm: round up tmpfs size to huge page size when huge=always

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Message-ID: <20171008125651.3mxiayuvuqi2hiku@node.shutemov.name>
Date:   Sun, 8 Oct 2017 15:56:51 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yang Shi <yang.s@...baba-inc.com>
Cc:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        mhocko@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: shm: round up tmpfs size to huge page size when
 huge=always

On Sat, Oct 07, 2017 at 04:22:10AM +0800, Yang Shi wrote:
> When passing "huge=always" option for mounting tmpfs, THP is supposed to
> be allocated all the time when it can fit, but when the available space is
> smaller than the size of THP (2MB on x86), shmem fault handler still tries
> to allocate huge page every time, then fallback to regular 4K page
> allocation, i.e.:
> 
> 	# mount -t tmpfs -o huge,size=3000k tmpfs /tmp
> 	# dd if=/dev/zero of=/tmp/test bs=1k count=2048
> 	# dd if=/dev/zero of=/tmp/test1 bs=1k count=2048
> 
> The last dd command will handle 952 times page fault handler, then exit
> with -ENOSPC.
> 
> Rounding up tmpfs size to THP size in order to use THP with "always"
> more efficiently. And, it will not wast too much memory (just allocate
> 511 extra pages in worst case).

Hm. I don't think it's good idea to silently increase size of fs.

Maybe better just refuse to mount with huge=always for too small fs?

-- 
 Kirill A. Shutemov