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Date:   Sun, 4 Jun 2023 22:48:47 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
        Wang Yugui <wangyugui@e16-tech.com>,
        Dave Chinner <david@fromorbit.com>,
        Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v2 5/7] filemap: Allow __filemap_get_folio to allocate
 large folios
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References: <20230602222445.2284892-1-willy@infradead.org>
 <20230602222445.2284892-6-willy@infradead.org>
 <20230604180925.GF72241@frogsfrogsfrogs>
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On Sun, Jun 04, 2023 at 11:09:25AM -0700, Darrick J. Wong wrote:
> On Fri, Jun 02, 2023 at 11:24:42PM +0100, Matthew Wilcox (Oracle) wrote:
> > +		do {
> > +			err = -ENOMEM;
> > +			if (order == 1)
> > +				order = 0;
> 
> Doesn't this interrupt the scale-down progression 2M -> 1M -> 512K ->
> 256K -> 128K -> 64K -> 32K -> 16K -> 4k?  What if I want 8k folios?

You can't have order-1 file/anon folios.  We have deferred_list in the
third page, so we have to have at least three pages in every large folio.
I forget exactly what it's used for; maybe there's a way to do without
it, but for now that's the rule.