On 27/07/2023 17:38, Yu Zhao wrote: > On Thu, Jul 27, 2023 at 1:26 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >> >> On 27/07/2023 03:35, Matthew Wilcox wrote: >>> On Thu, Jul 27, 2023 at 09:29:24AM +0800, Huang, Ying wrote: >>>> Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: >>>>> I think that can make sense. Because we limit to a single page table, >>>>> specifying 'nr = 1 << PMD_ORDER' is the same as 'compound = true'. >>>>> Just make it folio, page, nr, vma. I'd actually prefer it as (vma, >>>>> folio, page, nr), but that isn't the convention we've had in rmap up >>>>> until now. >>>> >>>> IIUC, even if 'nr = 1 << PMD_ORDER', we may remove one PMD 'compound' >>>> mapping, or 'nr' PTE mapping. So, we will still need 'compound' (or >>>> some better name) as parameter. >>> >>> Oh, this is removing ... so you're concerned with the case where we've >>> split the PMD into PTEs, but all the PTEs are still present in a single >>> page table? OK, I don't have a good answer to that. Maybe that torpedoes >>> the whole idea; I'll think about it. >> >> This is exactly why I think the approach I've already taken is the correct one; >> a 'range' makes no sense when you are dealing with 'compound' pages because you >> are accounting the entire folio. So surely its better to reflect that by only >> accounting small pages in the range version of the API. > > If the argument is the compound case is a separate one, then why not a > separate API for it? > > I don't really care about whether we think 'range' makes sense for > 'compound' or not. What I'm saying is: > 1. if they are considered one general case, then one API with the > compound parameter. > 2. if they are considered two specific cases, there should be two APIs. > This common design pattern is cleaner IMO. Option 2 definitely makes sense to me and I agree that it would be cleaner to have 2 separate APIs, one for small-page accounting (which can accept a range within a folio) and one for large-page accounting (i.e. compound=true in today's API). But... 1) That's not how the rest of the rmap API does it 2) This would be a much bigger change since I'm removing an existing API and replacing it with a completely new one (there are ~20 call sites to fix up). I was trying to keep the change small and manageable by maintaining the current API but moving all the small-page logic to the new API, so the old API is a wrapper in that case. 3) You would also need an API for the hugetlb case, which page_remove_rmap() handles today. Perhaps that could also be done by the new API that handles the compound case. But then you are mixing and matching your API styles - one caters for 1 specific case, and the other caters for 2 cases and figures out which one. > > Right now we have an overlap (redundancy) -- people would have to do > two code searches: one for page_remove_rmap() and the other for > folio_remove_rmap_range(nr=1), and this IMO is a bad design pattern. I'm open to doing the work to remove this redundancy, but I'd like to hear concensus on this thread that its the right approach first. Although personally I don't see a problem with what I've already done; If you want to operate on a page (inc the old concept of a "compound page" and a hugetlb page) call the old one. If you want to operate on a range of pages in a folio, call the new one. Thanks, Ryan
