Re: [PATCH 0/5] export __clear_page_buffers to cleanup code

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 20, 2020 at 11:14:35PM +0200, Guoqing Jiang wrote:
> On 20.04.20 02:30, Matthew Wilcox wrote:
> > On Mon, Apr 20, 2020 at 09:20:46AM +1000, Dave Chinner wrote:
> > > Anyone expecting to use set/clear_page_private as a matched pair (as
> > > the names suggest they are) is in for a horrible surprise...
> 
> Dave, thanks for the valuable suggestion!
> 
> > Oh, blast.  I hadn't noticed that.  And we're horribly inconsistent
> > with how we use set_page_private() too -- rb_alloc_aux_page() doesn't
> > increment the page's refcount, for example.
> > 
> > So, new (pair of) names:
> > 
> > set_fs_page_private()
> > clear_fs_page_private()
> 
> Hmm, maybe it is better to keep the original name (set/clear_page_private).

No.  Changing the semantics of set_page_private() without changing the
function signature is bad because it makes patches silently break when
applied to trees on either side of the change.  So we need a new name.

> 1. it would be weird for other subsystems (not belong to fs scope) to call
> the
> function which is supposed to be used in fs, though we can add a wrapper
> for other users out of fs.
> 
> 2. no function in mm.h is named like *fs*.

perhaps it should be in pagemap.h since it's for pagecache pages.

> > since it really seems like it's only page cache pages which need to
> > follow the rules about setting PagePrivate and incrementing the refcount.
> > Also, I think I'd like to see them take/return a void *:
> > 
> > void *set_fs_page_private(struct page *page, void *data)
> > {
> > 	get_page(page);
> > 	set_page_private(page, (unsigned long)data);
> > 	SetPagePrivate(page);
> > 	return data;
> > }
> 
> Seems  some functions could probably use the above helper, such as
> iomap_page_create, iomap_migrate_page, get_page_bootmem and
>  f2fs_set_page_private etc.

Yes.

> Really appreciate for your input though the thing is a little beyond my
> original intention ;-), will try to send a new version after reading more
> fs code.

That's kind of the way things go ... you start pulling on one thread
and all of a sudden, you're weaving a new coat ;-)