linux-kernel - Re: [PATCH 0/8] add mTHP support for anonymous shmem

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Date: Fri, 10 May 2024 11:53:51 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: David Hildenbrand <david@...hat.com>, Hugh Dickins <hughd@...gle.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Christoph Lameter <christoph@...eter.com>,
	Christoph Hellwig <hch@....de>, Dave Chinner <david@...morbit.com>,
	Daniel Gomez <da.gomez@...sung.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hughd@...gle.com" <hughd@...gle.com>,
	"ioworker0@...il.com" <ioworker0@...il.com>,
	"wangkefeng.wang@...wei.com" <wangkefeng.wang@...wei.com>,
	"ying.huang@...el.com" <ying.huang@...el.com>,
	"21cnbao@...il.com" <21cnbao@...il.com>,
	"ryan.roberts@....com" <ryan.roberts@....com>,
	"shy828301@...il.com" <shy828301@...il.com>,
	"ziy@...dia.com" <ziy@...dia.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 0/8] add mTHP support for anonymous shmem

On Thu, May 09, 2024 at 07:48:46PM +0200, David Hildenbrand wrote:
> On 08.05.24 21:23, Luis Chamberlain wrote:
> >  From my perspective the more shared code the better, and the more shared
> > paths the better. There is a chance to help test swap with large folios
> > instead of splitting the folios for swap, and that would could be done
> > first with tmpfs. I have not evaluated the difference in testing or how
> > we could get the most of shared code if we take a mTHP approach or the
> > iomap approach for tmpfs, that should be considered.
> 
> I don't have a clear picture yet of what might be best for ordinary shmem
> (IOW, not MAP_SHARED|MAP_PRIVATE), and I'm afraid there is no easy answer.

OK so it sounds like the different options needs to be thought out and
reviewed.

> As long as we don't end up wasting memory, it's not obviously bad.

Sure.

> But some
> things might be tricky (see my example about large folios stranding in shmem
> and never being able to be really reclaimed+reused for better purposes)

Where is that stated BTW? Could that be resolved?

> I'll note that mTHP really is just (supposed to be) a user interface to
> enable the various folio sizes (well, and to expose better per-size stats),
> not more.

Sure but given filesystems using large folios don't have silly APIs for
using which large folios to enable, it just seems odd for tmpfs to take
a different approach.

> From that point of view, it's just a filter. Enable all, and you get the
> same behavior as you likely would in the pagecache mode.

Which begs the quesiton, *why* have an API to just constrain to certain
large folios, which diverges from what filesystems are doing with large
folios?

> > Are there other things to consider? Does this require some dialog at
> > LSFMM?
> 
> As raised in my reply to Daniel, I'll be at LSF/MM and happy to discuss. I'm
> also not a SHMEM expert, so I'm hoping at some point we'd get feedback from
> Hugh.

Hugh, will you be at LSFMM?

  Luis