On 11 Jan 2026, at 19:19, Balbir Singh wrote: > On 1/12/26 08:35, Matthew Wilcox wrote: >> On Sun, Jan 11, 2026 at 09:55:40PM +0100, Francois Dugast wrote: >>> The core MM splits the folio before calling folio_free, restoring the >>> zone pages associated with the folio to an initialized state (e.g., >>> non-compound, pgmap valid, etc...). The order argument represents the >>> folio’s order prior to the split which can be used driver side to know >>> how many pages are being freed. >> >> This really feels like the wrong way to fix this problem. >>
Hi Matthew, I think the wording is confusing, since the actual issue is that: 1. zone_device_page_init() calls prep_compound_page() to form a large folio, 2. but free_zone_device_folio() never reverse the course, 3. the undo of prep_compound_page() in free_zone_device_folio() needs to be done before driver callback ->folio_free(), since once ->folio_free() is called, the folio can be reallocated immediately, 4. after the undo of prep_compound_page(), folio_order() can no longer provide the original order information, thus, folio_free() needs that for proper device side ref manipulation. So this is not used for "split" but undo of prep_compound_page(). It might look like a split to non core MM people, since it changes a large folio to a bunch of base pages. BTW, core MM has no compound_page_dctor() but open codes it in free_pages_prepare() by resetting page flags, page->mapping, and so on. So it might be why the undo prep_compound_page() is missed by non core MM people. > > This stems from a special requirement, freeing is done in two phases > > 1. Free the folio -> inform the driver (which implies freeing the backing > device memory) > 2. Return the folio back, split it back to single order folios Hi Balbir, Please refrain from using "split" here, since it confuses MM people. A folio is split when it is still in use, but in this case, the folio has been freed and needs to be restored to "free page" state. > > The current code does not do 2. 1 followed by 2 does not work for > Francois since the backing memory can get reused before we reach step 2. > The proposed patch does 2 followed 1, but doing 2 means we've lost the > folio order and thus the old order is passed in. Although, I wonder if the > backing folio's zone_device_data can be used to encode any order information > about the device side allocation. > > @Francois, I hope I did not miss anything in the explanation above. > >> I think someone from the graphics side really needs to take the lead on >> understanding what the MM is doing (both currently and in the future). >> I'm happy to work with you, but it feels like there's a lot of churn right >> now because there's a lot of people working on this without understanding >> the MM side of things (and conversely, I don't think (m)any people on the >> MM side really understand what graphics cards are trying to accomplish). >> > > I suspect you are referring to folio specialization and/or downsizing? > >> Who is that going to be? I'm happy to get on the phone with someone. > > Happy to work with you, but I am not the authority on graphics, I can speak > to zone device folios. I suspect we'd need to speak to more than one person. > -- Best Regards, Yan, Zi
