Re: [PATCH 10/10] mm/migrate.c: call detach_page_private to cleanup code

[Gao Xiang <hsiangkao@gmx.com>]

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On Tue, May 19, 2020 at 09:35:59AM +0200, Guoqing Jiang wrote:
> On 5/19/20 7:12 AM, Andrew Morton wrote:
> > On Sun, 17 May 2020 23:47:18 +0200 Guoqing Jiang <guoqing.jiang@cloud.ionos.com> wrote:
> >
> > > We can cleanup code a little by call detach_page_private here.
> > >
> > > ...
> > >
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -804,10 +804,7 @@ static int __buffer_migrate_page(struct address_space *mapping,
> > >   	if (rc != MIGRATEPAGE_SUCCESS)
> > >   		goto unlock_buffers;
> > > -	ClearPagePrivate(page);
> > > -	set_page_private(newpage, page_private(page));
> > > -	set_page_private(page, 0);
> > > -	put_page(page);
> > > +	set_page_private(newpage, detach_page_private(page));
> > >   	get_page(newpage);
> > >   	bh = head;
> > mm/migrate.c: In function '__buffer_migrate_page':
> > ./include/linux/mm_types.h:243:52: warning: assignment makes integer from pointer without a cast [-Wint-conversion]
> >   #define set_page_private(page, v) ((page)->private = (v))
> >                                                      ^
> > mm/migrate.c:800:2: note: in expansion of macro 'set_page_private'
> >    set_page_private(newpage, detach_page_private(page));
> >    ^~~~~~~~~~~~~~~~
> >
> > The fact that set_page_private(detach_page_private()) generates a type
> > mismatch warning seems deeply wrong, surely.
> >
> > Please let's get the types sorted out - either unsigned long or void *,
> > not half-one and half-the other.  Whatever needs the least typecasting
> > at callsites, I suggest.
>
> Sorry about that, I should notice the warning before. I will double check if
> other
> places need the typecast or not, then send a new version.
>
> > And can we please implement set_page_private() and page_private() with
> > inlined C code?  There is no need for these to be macros.
>
> Just did a quick change.
>
> -#define page_private(page)Â Â Â Â Â Â Â Â Â Â Â Â  ((page)->private)
> -#define set_page_private(page, v)Â Â Â Â Â  ((page)->private = (v))
> +static inline unsigned long page_private(struct page *page)
> +{
> +Â Â Â Â Â Â  return page->private;
> +}
> +
> +static inline void set_page_private(struct page *page, unsigned long
> priv_data)
> +{
> +Â Â Â Â Â Â  page->private = priv_data;
> +}
>
> Then I get error like.
>
> fs/erofs/zdata.h: In function ‘z_erofs_onlinepage_index’:
> fs/erofs/zdata.h:126:8: error: lvalue required as unary ‘&’ operand
> Â  u.v = &page_private(page);
> Â Â Â Â Â Â Â  ^
>
> I guess it is better to keep page_private as macro, please correct me in
> case I
> missed something.

I guess that you could Cc me in the reply.

In that case, EROFS uses page->private as an atomic integer to
trace 2 partial subpages in one page.

I think that you could also use &page->private instead directly to
replace &page_private(page) here since I didn't find some hint to
pick &page_private(page) or &page->private.


In addition, I found some limitation of new {attach,detach}_page_private
helper (that is why I was interested in this series at that time [1] [2],
but I gave up finally) since many patterns (not all) in EROFS are

io_submit (origin, page locked):
attach_page_private(page);
...
put_page(page);

end_io (page locked):
SetPageUptodate(page);
unlock_page(page);

since the page is always locked, so io_submit could be simplified as
set_page_private(page, ...);
SetPagePrivate(page);
, which can save both one temporary get_page(page) and one
put_page(page) since it could be regarded as safe with page locked.


btw, I noticed the patchset versions are PATCH [3], RFC PATCH [4],
RFC PATCH v2 [5], RFC PATCH v3 [6], PATCH [7]. Although I also
noticed the patchset title was once changed, but it could be some
harder to trace the whole history discussion.

[1] https://lore.kernel.org/linux-fsdevel/20200419051404.GA30986@hsiangkao-HP-ZHAN-66-Pro-G1/
[2] https://lore.kernel.org/linux-fsdevel/20200427025752.GA3979@hsiangkao-HP-ZHAN-66-Pro-G1/
[3] https://lore.kernel.org/linux-fsdevel/20200418225123.31850-1-guoqing.jiang@cloud.ionos.com/
[4] https://lore.kernel.org/linux-fsdevel/20200426214925.10970-1-guoqing.jiang@cloud.ionos.com/
[5] https://lore.kernel.org/linux-fsdevel/20200430214450.10662-1-guoqing.jiang@cloud.ionos.com/
[6] https://lore.kernel.org/linux-fsdevel/20200507214400.15785-1-guoqing.jiang@cloud.ionos.com/
[7] https://lore.kernel.org/linux-fsdevel/20200517214718.468-1-guoqing.jiang@cloud.ionos.com/

Thanks,
Gao Xiang

>
> Thanks,
> Guoqing
>
>
>