linux-kernel - Re: [PATCH 11/11] afs: Fix dirty-region encoding on ppc32 with 64K pages

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Message-ID: <541369.1603903981@warthog.procyon.org.uk>
Date:   Wed, 28 Oct 2020 16:53:01 +0000
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, linux-afs@...ts.infradead.org,
        kernel test robot <lkp@...el.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/11] afs: Fix dirty-region encoding on ppc32 with 64K pages

Matthew Wilcox <willy@...radead.org> wrote:

> > +static inline unsigned int afs_page_dirty_resolution(void)
> 
> I've been using size_t for offsets within a struct page.  I don't know
> that we'll ever support pages larger than 2GB (they're completely
> impractical with today's bus speeds), but I'd rather not be the one
> who has to track down all the uses of 'int' in the kernel in fifteen
> years time.

Going beyond 2G page size won't be fun and a lot of our APIs will break -
write_begin, write_end, invalidatepage to name a few.

It would probably require an analysis program to trace all the usages of such
information within the kernel.

> > +{
> > +	if (PAGE_SIZE - 1 <= __AFS_PAGE_PRIV_MASK)
> > +		return 1;
> > +	else
> > +		return PAGE_SIZE / (__AFS_PAGE_PRIV_MASK + 1);
> 
> Could this be DIV_ROUND_UP(PAGE_SIZE, __AFS_PAGE_PRIV_MASK + 1); avoiding
> a conditional?  I appreciate it's calculated at compile time today, but
> it'll be dynamic with THP.

That seems to work.

> >  static inline unsigned int afs_page_dirty_to(unsigned long priv)
> >  {
> > -	return ((priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK) + 1;
> > +	unsigned int x = (priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK;
> > +
> > +	/* The upper bound is exclusive */
> 
> I think you mean 'inclusive'.

The returned upper bound points immediately beyond the range.  E.g. 0-0 is an
empty range.  Changing that is way too big an overhaul outside the merge
window.

> > +	return (x + 1) * afs_page_dirty_resolution();
> >  }
> >  
> >  static inline unsigned long afs_page_dirty(unsigned int from, unsigned int to)
> >  {
> > +	unsigned int res = afs_page_dirty_resolution();
> > +	from /= res; /* Round down */
> > +	to = (to + res - 1) / res; /* Round up */
> >  	return ((unsigned long)(to - 1) << __AFS_PAGE_PRIV_SHIFT) | from;
> 
> Wouldn't it produce the same result to just round down?  ie:
> 
> 	to = (to - 1) / res;
> 	return ((unsigned long)to << __AFS_PAGE_PRIV_SHIFT) | from;

Actually, yes, because res/res==1, which I then subtract afterwards.

David