linux-kernel - Re: [PATCH v2 0/6] tmpfs: add the option to disable swap

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Message-ID: <ZBjR15Tr1o98crup@bombadil.infradead.org>
Date:   Mon, 20 Mar 2023 14:36:23 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     haoxin <xhao@...ux.alibaba.com>
Cc:     hughd@...gle.com, akpm@...ux-foundation.org, willy@...radead.org,
        brauner@...nel.org, linux-mm@...ck.org, p.raghav@...sung.com,
        da.gomez@...sung.com, a.manzanares@...sung.com, dave@...olabs.net,
        yosryahmed@...gle.com, keescook@...omium.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/6] tmpfs: add the option to disable swap

On Mon, Mar 20, 2023 at 07:14:22PM +0800, haoxin wrote:
> 
> 在 2023/3/20 上午4:32, Luis Chamberlain 写道:
> > On Tue, Mar 14, 2023 at 10:46:28AM +0800, haoxin wrote:
> > > All these series looks good to me and i do some test on my virtual machine
> > > it works well.
> > > 
> > > so please add Tested-by: Xin Hao<xhao@...ux.alibaba.com>  .
> > > 
> > > just one question, if tmpfs pagecache occupies a large amount of memory, how
> > > can we ensure successful memory reclamation in case of memory shortage?
> > If you're disabling swap then you know the only thing you can do is
> > unmount if you want to help the VM, otherwise the pressure is just
> > greater for the VM.
> 
> Un, what i mean is can we add a priority so that this type of pagecache is
> reclaimed last ?

That seems to be a classifier request for something much less aggressive
than mapping_set_unevictable(). My patches *prior* to using mapping_set_unevictable()
are I think closer to what it seems you want, but as noted before by
folks, that also puts unecessary stress on the VM because just fail
reclaim on our writepage().

> Instead of just setting the parameter noswap to make it unreclaimed, because
> if such pagecache which occupy big part of memory which can not be
> reclaimed, it will cause OOM.

You can't simultaneously retain possession of a cake and eat it, too,
once you eat it, its gone and noswap eats the cake because of the
suggestion / decision to follow through with mapping_set_unevictable().

It sounds like you want to make mapping_set_unevictable() optional and
deal with the possible stress incurred writepage() failing? Not quite
sure what else to recommend here.

  Luis