linux-kernel - Re: [PATCH v2 0/6] mm: page_type, zsmalloc and page_mapcount

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Date: Thu, 30 May 2024 14:02:23 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Mike Rapoport <rppt@...nel.org>, Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH v2 0/6] mm: page_type, zsmalloc and page_mapcount_reset()

On (24/05/29 13:18), David Hildenbrand wrote:
> Wanting to remove the remaining abuser of _mapcount/page_type along with
> page_mapcount_reset(), I stumbled over zsmalloc, which is yet to be
> converted away from "struct page" [1].
> 
> Unfortunately, we cannot stop using the page_type field in zsmalloc code
> completely for its own purposes. All other fields in "struct page" are
> used one way or the other. Could we simply store a 2-byte offset value
> at the beginning of each page? Likely, but that will require a bit more
> work; and once we have memdesc we might want to move the offset in there
> (struct zsalloc?) again.
> 
> ... but we can limit the abuse to 16 bit, glue it to a page type that
> must be set, and document it. page_has_type() will always successfully
> indicate such zsmalloc pages, and such zsmalloc pages only.
> 
> We lose zsmalloc support for PAGE_SIZE > 64KB, which should be tolerable.
> We could use more bits from the page type, but 16 bit sounds like a good
> idea for now.
> 
> So clarify the _mapcount/page_type documentation, use a proper page_type
> for zsmalloc, and remove page_mapcount_reset().
> 
> Briefly tested with zram on x86-64.
> 
> [1] https://lore.kernel.org/all/20231130101242.2590384-1-42.hyeyoo@gmail.com/
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
> Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>

Tested-by: Sergey Senozhatsky <senozhatsky@...omium.org> # zram/zsmalloc workloads