linux-kernel - Re: THP-enabled filesystem vs. FALLOC_FL_PUNCH

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Message-ID: <56D9C882.3040808@intel.com>
Date:	Fri, 4 Mar 2016 09:40:18 -0800
From:	Dave Hansen <dave.hansen@...el.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Yang Shi <yang.shi@...aro.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: THP-enabled filesystem vs. FALLOC_FL_PUNCH_HOLE

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?