linux-kernel - Re: [PATCH 01/17] mm/highmem: Lift memcpy_[to|from]_page and memset

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Message-ID: <20201127132006.GY4327@casper.infradead.org>
Date:   Fri, 27 Nov 2020 13:20:06 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, ira.weiny@...el.com,
        Dave Hansen <dave.hansen@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Eric Biggers <ebiggers@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Howells <dhowells@...hat.com>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Steve French <sfrench@...ba.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        Nicolas Pitre <nico@...xnic.net>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Brian King <brking@...ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 01/17] mm/highmem: Lift memcpy_[to|from]_page and
 memset_page to core

On Fri, Nov 27, 2020 at 03:06:24PM +0200, Joonas Lahtinen wrote:
> Quoting ira.weiny@...el.com (2020-11-24 08:07:39)
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > Working through a conversion to a call such as kmap_thread() revealed
> > many places where the pattern kmap/memcpy/kunmap occurred.
> > 
> > Eric Biggers, Matthew Wilcox, Christoph Hellwig, Dan Williams, and Al
> > Viro all suggested putting this code into helper functions.  Al Viro
> > further pointed out that these functions already existed in the iov_iter
> > code.[1]
> > 
> > Placing these functions in 'highmem.h' is suboptimal especially with the
> > changes being proposed in the functionality of kmap.  From a caller
> > perspective including/using 'highmem.h' implies that the functions
> > defined in that header are only required when highmem is in use which is
> > increasingly not the case with modern processors.  Some headers like
> > mm.h or string.h seem ok but don't really portray the functionality
> > well.  'pagemap.h', on the other hand, makes sense and is already
> > included in many of the places we want to convert.
> > 
> > Another alternative would be to create a new header for the promoted
> > memcpy functions, but it masks the fact that these are designed to copy
> > to/from pages using the kernel direct mappings and complicates matters
> > with a new header.
> > 
> > Lift memcpy_to_page(), memcpy_from_page(), and memzero_page() to
> > pagemap.h.
> > 
> > Also, add a memcpy_page(), memmove_page, and memset_page() to cover more
> > kmap/mem*/kunmap. patterns.
> > 
> > [1] https://lore.kernel.org/lkml/20201013200149.GI3576660@ZenIV.linux.org.uk/
> >     https://lore.kernel.org/lkml/20201013112544.GA5249@infradead.org/
> > 
> > Cc: Dave Hansen <dave.hansen@...el.com>
> > Suggested-by: Matthew Wilcox <willy@...radead.org>
> > Suggested-by: Christoph Hellwig <hch@...radead.org>
> > Suggested-by: Dan Williams <dan.j.williams@...el.com>
> > Suggested-by: Al Viro <viro@...iv.linux.org.uk>
> > Suggested-by: Eric Biggers <ebiggers@...nel.org>
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> 
> <SNIP>
> 
> > +static inline void memset_page(struct page *page, int val, size_t offset, size_t len)
> > +{
> > +       char *addr = kmap_atomic(page);
> > +       memset(addr + offset, val, len);
> > +       kunmap_atomic(addr);
> > +}
> 
> Other functions have (page, offset) pair. Insertion of 'val' in the middle here required
> to take a double look during review.

Let's be explicit here.  Your suggested order is:

	(page, offset, val, len)

right?  I think I would prefer that to (page, val, offset, len).