Subject: Re: THP-enabled filesystem vs. FALLOC_FL_PUNCH_HOLE
To: "Kirill A. Shutemov" <kirill@shutemov.name>, linux-fsdevel@vger.kernel.org,
        linux-api@vger.kernel.org
References: <1457023939-98083-1-git-send-email-kirill.shutemov@linux.intel.com>
 <20160304112603.GA9790@node.shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, Christoph Lameter <cl@gentwo.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Jerome Marchand <jmarchan@redhat.com>, Yang Shi <yang.shi@linaro.org>,
        Sasha Levin <sasha.levin@oracle.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <56D9C882.3040808@intel.com>
Date: Fri, 4 Mar 2016 09:40:18 -0800
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On 03/04/2016 03:26 AM, Kirill A. Shutemov wrote:
> On Thu, Mar 03, 2016 at 07:51:50PM +0300, Kirill A. Shutemov wrote:
>> Truncate and punch hole that only cover part of THP range is implemented
>> by zero out this part of THP.
>>
>> This have visible effect on fallocate(FALLOC_FL_PUNCH_HOLE) behaviour.
>> As we don't really create hole in this case, lseek(SEEK_HOLE) may have
>> inconsistent results depending what pages happened to be allocated.
>> Not sure if it should be considered ABI break or not.
> 
> Looks like this shouldn't be a problem. man 2 fallocate:
> 
> 	Within the specified range, partial filesystem blocks are zeroed,
> 	and whole filesystem blocks are removed from the file.  After a
> 	successful call, subsequent reads from this range will return
> 	zeroes.
> 
> It means we effectively have 2M filesystem block size.

The question is still whether this will case problems for apps.

Isn't 2MB a quote unusual block size?  Wouldn't some files on a tmpfs
filesystem act like they have a 2M blocksize and others like they have
4k?  Would that confuse apps?