linux-kernel - Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local

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Date:   Sat, 21 Jan 2023 19:26:56 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Helge Deller <deller@....de>, Matthew Wilcox <willy@...radead.org>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-parisc@...r.kernel.org
Subject: Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()

On Sat, Jan 21, 2023 at 12:05:58AM -0800, Ira Weiny wrote:

> First, arn't PAGE_ALIGN_DOWN(addr) and PTR_ALIGN_DOWN(addr, PAGE_SIZE) the
> same?
> 
> align.h
> #define PTR_ALIGN_DOWN(p, a)    ((typeof(p))ALIGN_DOWN((unsigned long)(p), (a)))
> 
> mm.h:
> #define PAGE_ALIGN_DOWN(addr) ALIGN_DOWN(addr, PAGE_SIZE)

... and ALIGN_DOWN ends up with doing bitwise and on the first argument.
Which doesn't work for pointers, thus the separate variant for those
and typecast to unsigned long in it...