On Tue, Feb 18, 2014 at 10:28:11AM -0800, Linus Torvalds wrote:

On Tue, Feb 18, 2014 at 10:07 AM, Kirill A. Shutemov
[off-list ref] wrote:

quoted

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?

I don't like this too...

Current max_pgoff is end of page table (or end of vma, if it ends before).

If we drop nr_pages but keep current max_pgoff, we will potentially setup
PTRS_PER_PTE pages a time: i.e. page fault to first page of page table and
all pages are ready. nr_pages limits the number.

It's not necessary bad idea to populate whole page table at once. I need
to measure how much latency we will add by doing that.

The only problem I see is that we take ptl for a bit too long. But with
split ptl it will affect only page table we populate.

Other approach is too limit ourself to FAULT_AROUND_PAGES from start_addr.
In this case sometimes we will do useless radix-tree lookup even if we had
chance to populated pages further in the page table.

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.

The code has not ready yet. I'll rework it. It just what I had by the end
of the day. I wanted to know if setup pte directly from ->fault_nonblock()
is okayish approach or considered layering violation.

-- 
 Kirill A. Shutemov

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