linux-kernel - Re: [GIT PULL] Memory folios for v5.15

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Date:   Wed, 1 Sep 2021 15:58:37 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] Memory folios for v5.15

On Tue, Aug 24, 2021 at 03:44:48PM -0400, Theodore Ts'o wrote:
> On Tue, Aug 24, 2021 at 08:23:15PM +0100, Matthew Wilcox wrote:
>
> So if someone sees "kmem_cache_alloc()", they can probably make a
> guess what it means, and it's memorable once they learn it.
> Similarly, something like "head_page", or "mempages" is going to a bit
> more obvious to a kernel newbie.  So if we can make a tiny gesture
> towards comprehensibility, it would be good to do so while it's still
> easier to change the name.

Talking about being newbie friendly, how about we'll just add a piece of
documentation along with the new type for a change?

Something along those lines (I'm sure willy can add several more sentences
for Folio description)

diff --git a/Documentation/vm/memory-model.rst b/Documentation/vm/memory-model.rst
index 30e8fbed6914..b5b39ebe67cf 100644
--- a/Documentation/vm/memory-model.rst
+++ b/Documentation/vm/memory-model.rst
@@ -30,6 +30,29 @@ Each memory model defines :c:func:`pfn_to_page` and :c:func:`page_to_pfn`
 helpers that allow the conversion from PFN to `struct page` and vice
 versa.
 
+Pages
+-----
+
+Each physical page frame in the system is represented by a `struct page`.
+This structure aggregatates several types, each corresponding to a
+particular usage of a page frame, such as anonymous memory, SLAB caches,
+file-backed memory etc. These types are define within unions in the struct
+page to reduce memory footprint of the memory map.
+
+The actual type of the particular insance of struct page is determined by
+values of the fields shared between the different types and can be quired
+using page flag operatoins defined in ``include/linux/page-flags.h``
+
+Folios
+------
+
+For many use cases, single page frame granularity is too small. In such
+cases a contiguous range of memory can be referred by `struct folio`.
+
+A folio is a physically, virtually and logically contiguous range of
+bytes. It is a power-of-two in size, and it is aligned to that same
+power-of-two. It is at least as large as PAGE_SIZE.
+
 FLATMEM
 =======

-- 
Sincerely yours,
Mike.