Re: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Feb 18, 2014 at 10:07 AM, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> Patch is wrong. Correct one is below.

Hmm. I don't hate this. Looking through it, it's fairly simple
conceptually, and the code isn't that complex either. I can live with
this.

I think it's a bit odd how you pass both "max_pgoff" and "nr_pages" to
the fault-around function, though. In fact, I'd consider that a bug.
Passing in "FAULT_AROUND_PAGES" is just wrong, since the code cannot -
and in fact *must* not - actually fault in that many pages, since the
starting/ending address can be limited by other things.

So I think that part of the code is bogus. You need to remove
nr_pages, because any use of it is just incorrect. I don't think it
can actually matter, since the max_pgoff checks are more restrictive,
but if you think it can matter please explain how and why it wouldn't
be a major bug?

Apart from that, I'd really like to see numbers for different ranges
of FAULT_AROUND_ORDER, because I think 5 is pretty high, but on the
whole I don't find this horrible, and you still lock the page so it
doesn't involve any new rules. I'm not hugely happy with another raw
radix-tree user, but it's not horrible.

Btw, is the "radix_tree_deref_retry(page) -> goto restart" really
necessary? I'd be almost more inclined to just make it just do a
"break;" to break out of the loop and stop doing anything clever at
all.

IOW, from a quick look there's a couple of small details I don't like
that look odd, but ..

                Linus

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