On 6/2/21 7:58 PM, Matthew Wilcox wrote:
On Wed, Jun 02, 2021 at 11:27:47AM +0800, Yu Xu wrote:
On 6/2/21 3:10 AM, Hugh Dickins wrote:
On Tue, 1 Jun 2021, Matthew Wilcox wrote:
On Tue, Jun 01, 2021 at 09:55:56AM -0700, Hugh Dickins wrote:
Well caught: you're absolutely right that there's a bug there.
But isn't cond_resched() just papering over the real bug, and
what it should do is a "page = compound_head(page);" before the
get_page_unless_zero()? How does that work out in your testing?
You do realise you're strengthening my case for folios by suggesting
that, don't you? ;-)
Hah! Well, I do realize that I'm offering you a marketing opportunity.
And you won't believe how many patches I dread to post for fear of that ;-)
But I'm not so sure that it strengthens your case: apparently folios
had not detected this? Or do you have a hoard of folio-detected fixes
waiting for the day, and a folio-kit for each of the stable releases?
I was going to suggest that it won't make any difference because the
page reference count is frozen, but the freezing happens after the call
to unmap_page(), so it may make a difference.
I think that's a good point: I may have just jumped on the missing
compound_head(), without thinking it through as far as you have.
I'm having trouble remembering the dynamics now; but I think there
are cond_resched()s in the unmap_page() part, so the splitter may
get preempted even on a non-preempt kernel; whereas the frozen
part is all done expeditiously, with interrupts disabled.
Greg discovered the same issue recently, but we all got sidetracked,
and I don't know where his investigation ended up. He was in favour
of cond_resched(), I was in favour of compound_head(); and I think I
I ever considered about using compound_head, but isn't there another
race that, the following put_and_wait_on_page_locked operates on the
"tail page" which has been split and is now a single page?
No, having your own reference on a page prevents the page from being
split. But that's a good question to ask.
Thanks. I have recalled that head page is frozen when splitting THP.
--
Thanks,
Yu
