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

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Message-ID: <20171009064811.lmotdeuewfbznhzq@dhcp22.suse.cz>
Date:   Mon, 9 Oct 2017 08:48:11 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Yang Shi <yang.s@...baba-inc.com>, kirill.shutemov@...ux.intel.com,
        hughd@...gle.com, 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 Sun 08-10-17 15:56:51, Kirill A. Shutemov wrote:
> 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.

Agreed!

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

We cannot we simply have the remaining page !THP? What is the actual
problem?
-- 
Michal Hocko
SUSE Labs