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From:   David Howells <dhowells@redhat.com>
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References: <YFja/LRC1NI6quL6@cmpxchg.org> <20210320054104.1300774-1-willy@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     dhowells@redhat.com,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-cachefs@redhat.com,
        linux-afs@lists.infradead.org
Subject: Re: [PATCH v5 00/27] Memory Folios
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Date:   Tue, 23 Mar 2021 17:50:31 +0000
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Johannes Weiner <hannes@cmpxchg.org> wrote:

> So I fully agree with the motivation behind this patch. But I do
> wonder why it's special-casing the commmon case instead of the rare
> case. It comes at a huge cost. Short term, the churn of replacing
> 'page' with 'folio' in pretty much all instances is enormous.
> 
> And longer term, I'm not convinced folio is the abstraction we want
> throughout the kernel. If nobody should be dealing with tail pages in
> the first place, why are we making everybody think in 'folios'? Why
> does a filesystem care that huge pages are composed of multiple base
> pages internally? This feels like an implementation detail leaking out
> of the MM code. The vast majority of places should be thinking 'page'
> with a size of 'page_size()'. Including most parts of the MM itself.

I like the idea of logically separating individual hardware pages from
abstract bundles of pages by using a separate type for them - at least in
filesystem code.  I'm trying to abstract some of the handling out of the
network filesystems and into a common library plus ITER_XARRAY to insulate
those filesystems from the VM.

David