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

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Date:	Mon, 7 Mar 2016 10:03:36 +1100
From:	Dave Chinner <david@...morbit.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 Sun, Mar 06, 2016 at 03:30:34AM +0300, Kirill A. Shutemov wrote:
> On Sun, Mar 06, 2016 at 09:38:11AM +1100, Dave Chinner wrote:
> > On Sat, Mar 05, 2016 at 02:24:12AM +0300, Kirill A. Shutemov wrote:
> > > Would it be acceptable for fallocate(FALLOC_FL_PUNCH_HOLE) to return
> > > -EBUSY (or other errno on your choice), if we cannot split the page
> > > right away?
> > 
> > Which means THP are not transparent any more. What does an
> > application do when it gets an EBUSY, anyway?
> 
> I guess it's reasonable to expect from an application to handle EOPNOTSUPP
> as FALLOC_FL_PUNCH_HOLE is not supported by some filesystems.

Yes, but this is usually done as a check at the program
initialisation to determine whether to issue hole punches at all.
It's not suppose to be a dynamic error.

> Although, non-consistent result from the same fd can be confusing.

Exactly.

> > And it's not just hole punching that has this problem. Direct IO is
> > going to have the same issue with invalidation of the mapped ranges
> > over the IO being done. XFS already WARNs when page cache
> > invalidation fails with EBUSY in direct IO, because that is
> > indicative of an application with a potential data corruption vector
> > and there's nothing we can do in the kernel code to prevent it.
> 
> My current understanding is that for filesystems with persistent storage,
> in order to make THP any useful, we would need to implement writeback
> without splitting the huge page.

Algorithmically it is no different to filesytem block size < page
size writeback.

> At the moment, I have no idea how hard it would be..

THP support would effectively require us to remove PAGE_CACHE_SIZE
assumptions from all of the filesystem and buffer code. That's a
large chunk of work e.g.  fs/buffer.c and any filesystem that uses
bufferheads for tracking filesystem block state through the page
cache.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com