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. 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
