On Mon, Jan 09, 2023 at 08:30:43AM -0800, Dave Hansen wrote: > On 1/9/23 05:24, Matthew Wilcox wrote: > > On Mon, Jan 09, 2023 at 03:22:29PM +0800, Yin Fengwei wrote: > >> The idea of the multiple consecutive page (abbr as "mcpage") is using > >> collection of physical contiguous 4K page other than huge page for > >> anonymous mapping. > > This is what folios are for. You have an interesting demonstration > > here that shows that moving to larger folios for anonymous memory > > is worth doing (thank you!) but you're missing several of the advantages > > of folios by going off and doing your own thing. > > It might not have come across in the changelog and cover letter, but > Fengwei and the rest of us *totally* agree with you on this. "Doing > your own thing" just isn't going to cut it and if this is going to go > anywhere, it needs to use folios. > > This series is _pure_ RFC and the comments we're interested in is > whether this demonstration warrants going back and doing it the right > way (with folios). Ah, yes, that didn't come across. In fact, the opposite came across with this paragraph: : This series is the first step of mcpage. The furture work can be : enable mcpage for more components like page cache, swapping etc. : Finally, most pages in system will be allocated/free/reclaimed : with mcpage order. Since the page cache has been using multipage folios in mainline since March (and in various trees of mine since Feb 2020!), that indicated to me either a lack of knowledge of folios, or a rejection of the folio approach. Happy to hear that's not true! Most of the results here validate my experience and/or assumptions, which is good. I'm more than happy for someone else to take on the hard work of folio-ising the anon VMAs. I see the problems to be solved as: - Determining (on a page fault) what the correct allocation size is for this process at this time. We have the readahead code to leverage for files, but I don't think we have anything similar for anon memory - Inserting multiple PTEs when a multi-page folio is found. This also needs to be done for file pages, and maybe that's a good place to start. - Finding all the places in the anon memory code that assume that PageCompound() / PageTransHuge() is the same thing as folio_test_pmd_mappable(). There are probably other things that are going to come up, but I think starting is the important part. Not everything needs to be done immediately (#3 before #1, I would think ;-).
