linux-ext4 - Re: [PATCH 01/15] mm: add invalidatepage

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Message-ID: <alpine.LSU.2.00.1208211144550.2178@eggly.anvils>
Date:	Tue, 21 Aug 2012 11:59:42 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Lukas Czerner <lczerner@...hat.com>
cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	tytso@....edu, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 01/15] mm: add invalidatepage_range address space
 operation

On Tue, 21 Aug 2012, Lukas Czerner wrote:
> On Sun, 19 Aug 2012, Hugh Dickins wrote:
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -620,6 +620,8 @@ struct address_space_operations {
> > >  	/* Unfortunately this kludge is needed for FIBMAP. Don't use it */
> > >  	sector_t (*bmap)(struct address_space *, sector_t);
> > >  	void (*invalidatepage) (struct page *, unsigned long);
> > > +	void (*invalidatepage_range) (struct page *, unsigned long,
> > > +				      unsigned long);
> > 
> > It may turn out to be bad advice, given how invalidatepage() already
> > takes an unsigned long, but I'd be tempted to make both of these args
> > unsigned int, since that helps to make it clearer that they're intended
> > to be offsets within a page, in the range 0..PAGE_CACHE_SIZE.
> > 
> > (partial_start, partial_end and top in truncate_inode_pages_range()
> > are all unsigned int.)
> 
> Hmm, this does not seem right. I can see that PAGE_CACHE_SIZE
> (PAGE_SIZE) can be defined as unsigned long, or am I missing
> something ?

They would be defined as unsigned long so that they can be used in
masks like ~(PAGE_SIZE - 1), and behave as expected on addresses,
without needing casts to be added all over.

We do not (currently!) expect PAGE_SIZE or PAGE_CACHE_SIZE to grow
beyond an unsigned int - but indeed they can be larger than what's
held in an unsigned short (look no further than ia64 or ppc64).

For more reassurance, see include/linux/highmem.h, which declares
zero_user_segments() and others: unsigned int (well, unsigned with
the int implicit) for offsets within a page.

Hugh

> > 
> > Andrew is very keen on naming arguments in prototypes,
> > and I think there is an especially strong case for it here.
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