On 12 Jan 2026, at 16:49, Matthew Brost wrote: > On Mon, Jan 12, 2026 at 09:45:10AM -0400, Jason Gunthorpe wrote: > > Hi, catching up here. > >> On Sun, Jan 11, 2026 at 07:51:01PM -0500, Zi Yan wrote: >>> 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. >> >> There is something wrong with the driver if the "folio can be >> reallocated immediately". >> >> The flow generally expects there to be a driver allocator linked to >> folio_free() >> >> 1) Allocator finds free memory >> 2) zone_device_page_init() allocates the memory and makes refcount=1 >> 3) __folio_put() knows the recount 0. >> 4) free_zone_device_folio() calls folio_free(), but it doesn't >> actually need to undo prep_compound_page() because *NOTHING* can >> use the page pointer at this point. > > Correct—nothing can use the folio prior to calling folio_free(). Once > folio_free() returns, the driver side is free to immediately reallocate > the folio (or a subset of its pages). > >> 5) Driver puts the memory back into the allocator and now #1 can >> happen. It knows how much memory to put back because folio->order >> is valid from #2 >> 6) #1 happens again, then #2 happens again and the folio is in the >> right state for use. The successor #2 fully undoes the work of the >> predecessor #2. >> >> If you have races where #1 can happen immediately after #3 then the >> driver design is fundamentally broken and passing around order isn't >> going to help anything. >> > > The above race does not exist; if it did, I agree we’d be solving > nothing here. > >> If the allocator is using the struct page memory then step #5 should >> also clean up the struct page with the allocator data before returning >> it to the allocator. >> > > We could move the call to free_zone_device_folio_prepare() [1] into the > driver-side implementation of ->folio_free() and drop the order argument > here. Zi didn’t particularly like that; he preferred calling > free_zone_device_folio_prepare() [2] before invoking ->folio_free(), > which is why this patch exists.
On a second thought, if calling free_zone_device_folio_prepare() in ->folio_free() works, feel free to do so. > > FWIW, I do not have a strong opinion here—either way works. Xe doesn’t > actually need the order regardless of where > free_zone_device_folio_prepare() is called, but Nouveau does need the > order if free_zone_device_folio_prepare() is called before > ->folio_free(). > > [1] https://patchwork.freedesktop.org/patch/697877/?series=159120&rev=4 > [2] > https://patchwork.freedesktop.org/patch/697709/?series=159120&rev=3#comment_1282405 > >> I vaugely remember talking about this before in the context of the Xe >> driver.. You can't just take an existing VRAM allocator and layer it >> on top of the folios and have it broadly ignore the folio_free >> callback. >> > > We are definitely not ignoring the ->folio_free callback—that is the > point at which we tell our VRAM allocator (DRM buddy) it is okay to > release the allocation and make it available for reuse. > > Matt > >> Jsaon Best Regards, Yan, Zi
