Date: Sun, 17 Apr 2016 03:46:26 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andres Lagar-Cavilla <andreslc@google.com>,
        Yang Shi <yang.shi@linaro.org>, Ning Qu <quning@gmail.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        kernel test robot <xiaolong.ye@intel.com>,
        Xiong Zhou <jencce.kernel@gmail.com>,
        Matthew Wilcox <willy@linux.intel.com>,
        Greg Thelen <gthelen@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH mmotm 5/5] huge tmpfs: add shmem_pmd_fault()
Message-ID: <20160417004626.GA5169@node.shutemov.name>
References: <alpine.LSU.2.11.1604161621310.1907@eggly.anvils>
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On Sat, Apr 16, 2016 at 04:41:33PM -0700, Hugh Dickins wrote:
> The pmd_fault() method gives the filesystem an opportunity to place
> a trans huge pmd entry at *pmd, before any pagetable is exposed (and
> an opportunity to split it on COW fault): now use it for huge tmpfs.
> 
> This patch is a little raw: with more time before LSF/MM, I would
> probably want to dress it up better - the shmem_mapping() calls look
> a bit ugly; it's odd to want FAULT_FLAG_MAY_HUGE and VM_FAULT_HUGE just
> for a private conversation between shmem_fault() and shmem_pmd_fault();
> and there might be a better distribution of work between those two, but
> prising apart that series of huge tests is not to be done in a hurry.
> 
> Good for now, presents the new way, but might be improved later.
> 
> This patch still leaves the huge tmpfs map_team_by_pmd() allocating a
> pagetable while holding page lock, but other filesystems are no longer
> doing so; and we've not yet settled whether huge tmpfs should (like anon
> THP) or should not (like DAX) participate in deposit/withdraw protocol.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Just for record: I don't like ->pmd_fault() approach because it results in
two requests to file system (two shmem_fault() in this case) if we don't
have a huge page to map: one for huge page (failed) and then one for small.
I think this case should be rather common: all mounts without huge pages
enabled. I expect performance regression from this too.

-- 
 Kirill A. Shutemov