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.
