Re: data loss in 2.6.9-rc1-mm1

[Nick Piggin]

Ram Pai wrote:
> On Fri, 2004-08-27 at 21:35, Nick Piggin wrote:
>
> > Ram Pai wrote:
> >
> > > got it!  Everything got changed to the new convention except that
> > > the calculation of 'nr' just before the check "nr <= offset" .
> > >
> > > I have generated this patch which takes care of that and hence fixes the
> > > data loss problem as well. I guess it is cleaner too. 
> > >
> > > This patch is generated w.r.t 2.6.8.1. If everybody blesses this patch I
> > > will forward it to Andrew.
> >
> > It looks like it should be OK... but at what point does it become
> > simpler to use my patch which just moves the original calculation
> > up, and does it again if we have to ->readpage()?
> >
> > (assuming you agree that it solves the problem)
>
>
> I agree your patch also solves the problem.
>
> Either way is fine. Even Hugh's patch almost does the same thing as
> yours.

Ahh, yep - Hugh just forgot to also move the "nr" calculation
into the ->readpage path, so it hits twice on the fast path.

> The only advantage with my page is it does the calculation in
> only one place and does not repeat it. Also I feel its more intuitive to

Well kind of - but you are having to jump through hoops to get there.
Yours does the following checks:

	/* fast path, read nr_pages from pagecache */
	if (!isize)
		goto out;
	for (i = 0; i < nr_pages; i++) {
		if (index > end_index)
			goto out;
		if (index == end_index) {
			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
			if (nr <= offset) {
				page_cache_release(page);
				goto out;
			}
		}

		/* slowpath, ->readpage */
		if (unlikely(!isize || index > end_index)) {
			page_cache_release(page);
			goto out;
		}
	}


Mine does:
	if (index > end_index)
		goto out;
	for (i = 0; i < pages_to_read; i++) {
		if (index == end_index) {
			nr = isize & ~PAGE_CACHE_MASK;
			if (nr <= offset)
				goto out;
		}

		/* slowpath, ->readpage */
		if (index > end_index) {
			page_cache_release(page);
			goto out;
		}
		if (index == end_index) {
			nr = isize & ~PAGE_CACHE_MASK;
			if (nr <= offset) {
				page_cache_release(page);
				goto out;
			}
		}
	}

So my fastpath is surely leaner, while the slowpath isn't a clear loser.

What's more, it looks like mine handles the corner case of reading off the
end of a non-PAGE_SIZE file (but within the same page). I think yours will
drop through and do the ->readpage, while mine doesn't...?


> assume that index 0 covers range 0 to 4095 i.e index n covers range
> n*PAGE_SIZE to ((n+1)*PAGE_SIZE)-1.  Currently the code assumes index 0
> covers range 1 to 4096  i.e index n covers range (n*PAGE_SIZE)+1 to
> (n+1)*PAGE_SIZE. 

It is definitely a pretty ugly function all round. I like the 0-4095 thing
better too, but my counter argument to that is that this is the minimal
change, and similar to how it has previously worked.

> this is the 4th time we are trying to nail down the same thing. We
> better get it right this time. So any correct patch is ok with me.

I agree. We'll leave it to someone else to decide, then ;)
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