Differential D48743
linuxkpi: Add `folio` and `folio_batch` APIs Authored by dumbbell on Jan 31 2025, 2:08 PM. Tags None Referenced Files
Details They are used by the i915 DRM driver in Linux 6.6 (although this change was only backported with Linux 6.7 DRM drivers...). struct folio simply wraps struct page for now. struct folio_batch is the same as struct pagevec but it works with struct folio instead of struct page directly. This is part of the update of DRM drivers to Linux 6.7. Sponsored by: The FreeBSD Foundation
Diff Detail
Event TimelineComment Actions Add diff context. No other changes. Manu told me the same, so while I’m uploading patches to address bz feedback, I’m updating existing patch with their context.
Comment Actions I spent some time to review the folio code and the existing page allocation code (alloc_pages(), __free_pages(), put_page(), release_pages(), ...). Pages from alloc_pages() are not attached to a VM object (unmanaged). Our code assumes that a page allocated this way will be freed using __free_pages() which takes care of the unwiring and if 0, caliing vm_page_free(). put_page() and release_pages() assume that the page was allocated by something else and pages are managed and that something else will take care of the page free. They don’t do anything special when the wiring count reaches zero. Now from folio_batch_release(), I wonder if I should check the VPO_MANAGED flag to determine if the page should simply be unwired (same as put_page()) or if it should call __free_pages(). In other words, I wonder if this flag can be used to unify the put_page(), release_pages() and folio_batch_release() functions. FYI, on Linux, put_page() calls folio_put() (which I will add) and folio_batch_release() calls release_pages() (which accept a struct page ** or a struct folio ** through an anonymous union and assumes that a page can be casted to a folio). When the refcount reaches zero, both code paths call the same code to free pages. Comment Actions I reworked the patch to unify the put_page() and __free_pages() code paths. Now, all put/freed pages are handled by __free_pages(). release_pages() accepts either a list of struct page or a list of struct folio, like Linux. Comment Actions I would be tempted to break out the move of release_pages() w/o modifications into an earlier commit. Likewise depending on outcome of the review the adjustments for put_page etc. (but that's partially beyond me; I hope markj or kib can help). Then the remaining bits a pure folio.
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