linux-kernel - Re: [PATCH v4 01/25] mm: Introduce struct folio

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Message-ID: <20210319012527.GX3420@casper.infradead.org>
Date:   Fri, 19 Mar 2021 01:25:27 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 01/25] mm: Introduce struct folio

On Fri, Mar 19, 2021 at 10:56:45AM +1100, Balbir Singh wrote:
> On Fri, Mar 05, 2021 at 04:18:37AM +0000, Matthew Wilcox (Oracle) wrote:
> > A struct folio refers to an entire (possibly compound) page.  A function
> > which takes a struct folio argument declares that it will operate on the
> > entire compound page, not just PAGE_SIZE bytes.  In return, the caller
> > guarantees that the pointer it is passing does not point to a tail page.
> >
> 
> Is this a part of a larger use case or general cleanup/refactor where
> the split between page and folio simplify programming?

The goal here is to manage memory in larger chunks.  Pages are now too
small for just about every workload.  Even compiling the kernel sees a 7%
performance improvement just by doing readahead using relatively small
THPs (16k-256k).  You can see that work here:
https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/master

I think Kirill, Hugh and others have done a fantastic job stretching
the page struct to work in shmem, but we really need a different type
to avoid people writing code that _looks_ right but is actually buggy.
So I'm starting again, this time with the folio metaphor.