On Thu, Oct 22, 2020 at 09:14:35AM +1100, Dave Chinner wrote: > On Tue, Oct 20, 2020 at 11:53:31PM +0100, Matthew Wilcox wrote: > > True, we don't _have to_ split THP on holepunch/truncation/... but it's > > a better implementation to free pages which cover blocks that no longer > > have data associated with them. > > "Better" is a very subjective measure. What numbers do you have > to back that up? None. When we choose to use a THP, we're choosing to treat a chunk of a file as a single unit for the purposes of tracking dirtiness, age, membership of the workingset, etc. We're trading off reduced precision for reduced overhead; just like the CPU tracks dirtiness on a cacheline basis instead of at byte level. So at some level, we've making the assumption that this 128kB THP is all one thingand it should be tracked together. But the user has just punched a hole in it. I can think of no stronger signal to say "The piece before this hole, the piece I just got rid of and the piece after this are three separate pieces of the file". If I could split them into pieces that weren't single pages, I would. Zi Yan has a patch to do just that, and I'm very much looking forward to that being merged. But saying "Oh, this is quite small, I'll keep the rest of the THP together" is conceptually wrong. > > Splitting the page instead of throwing it away makes sense once we can > > transfer the Uptodate bits to each subpage. If we don't have that, > > it doesn't really matter which we do. > > Sounds like more required functionality... I'm not saying that my patchset is the last word and there will be no tweaking. I'm saying I think it's good enough, an improvement on the status quo, and it's better to merge it for 5.11 than to keep it out of tree for another three months while we tinker with improving it. Do you disagree?
