Date: Fri, 4 Mar 2016 14:26:03 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org
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>,
        Dave Hansen <dave.hansen@intel.com>, 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
Subject: THP-enabled filesystem vs. FALLOC_FL_PUNCH_HOLE
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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.

And I don't see any guarantee about subsequent lseek(SEEK_HOLE) beheviour.

-- 
 Kirill A. Shutemov