| Message ID | 20241108162040.159038-1-tabba@google.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | mm: Introduce and use folio_owner_ops | expand |
On Fri, Nov 08, 2024 at 04:20:30PM +0000, Fuad Tabba wrote: > Some folios, such as hugetlb folios and zone device folios, > require special handling when the folio's reference count reaches > 0, before being freed. Moreover, guest_memfd folios will likely > require special handling to notify it once a folio's reference > count reaches 0, to facilitate shared to private folio conversion > [*]. Currently, each usecase has a dedicated callback when the > folio refcount reaches 0 to that effect. Adding yet more > callbacks is not ideal. Honestly, I question this thesis. How complex would it be to have 'yet more callbacks'? Is the challenge really that the mm can't detect when guestmemfd is the owner of the page because the page will be ZONE_NORMAL? So the point of this is really to allow ZONE_NORMAL pages to have a per-allocator callback? But this is also why I suggested to shift them to ZONE_DEVICE for guestmemfd, because then you get these things for free from the pgmap. (this is not a disagreement this is a valid solution, but a request you explain much more about what it is you actually need and compare it with the other existing options) Jason
On 08.11.24 18:05, Jason Gunthorpe wrote: > On Fri, Nov 08, 2024 at 04:20:30PM +0000, Fuad Tabba wrote: >> Some folios, such as hugetlb folios and zone device folios, >> require special handling when the folio's reference count reaches >> 0, before being freed. Moreover, guest_memfd folios will likely >> require special handling to notify it once a folio's reference >> count reaches 0, to facilitate shared to private folio conversion >> [*]. Currently, each usecase has a dedicated callback when the >> folio refcount reaches 0 to that effect. Adding yet more >> callbacks is not ideal. > Thanks for having a look! Replying to clarify some things. Fuad, feel free to add additional information. > Honestly, I question this thesis. How complex would it be to have 'yet > more callbacks'? Is the challenge really that the mm can't detect when > guestmemfd is the owner of the page because the page will be > ZONE_NORMAL? Fuad might have been a bit imprecise here: We don't want an ever growing list of checks+callbacks on the page freeing fast path. This series replaces the two cases we have by a single generic one, which is nice independent of guest_memfd I think. > > So the point of this is really to allow ZONE_NORMAL pages to have a > per-allocator callback? To intercept the refcount going to zero independent of any zones or magic page types, without as little overhead in the common page freeing path. It can be used to implement custom allocators, like factored out for hugetlb in this series. It's not necessarily limited to that, though. It can be used as a form of "asynchronous page ref freezing", where you get notified once all references are gone. (I might have another use case with PageOffline, where we want to prevent virtio-mem ones of them from getting accidentally leaked into the buddy during memory offlining with speculative references -- virtio_mem_fake_offline_going_offline() contains the interesting bits. But I did not look into the dirty details yet, just some thought where we'd want to intercept the refcount going to 0.) > > But this is also why I suggested to shift them to ZONE_DEVICE for > guestmemfd, because then you get these things for free from the pgmap. With this series even hugetlb gets it for "free", and hugetlb is not quite the nail for the ZONE_DEVICE hammer IMHO :) For things we can statically set aside early during boot and never really want to return to the buddy/another allocator, I would agree that static ZONE_DEVICE would have possible. Whenever the buddy or other allocators are involved, and we might have granularity as a handful of pages (e.g., taken from the buddy), getting ZONE_DEVICE involved is not a good (or even feasible) approach. After all, all we want is intercept the refcount going to 0.
Some folios, such as hugetlb folios and zone device folios, require special handling when the folio's reference count reaches 0, before being freed. Moreover, guest_memfd folios will likely require special handling to notify it once a folio's reference count reaches 0, to facilitate shared to private folio conversion [*]. Currently, each usecase has a dedicated callback when the folio refcount reaches 0 to that effect. Adding yet more callbacks is not ideal. This patch series introduces struct folio_owner_ops and uses it as a generic way to handle callbacks on freeing a folio. It also applies the callbacks to hugetlb and zone device folios. A pointer to struct folio_owner_ops is overlaid on struct page compound_page, struct page/folio lru. To indicate that the folio uses the callback, this patch series sets bit 1 of the new field, similar to how bit 0 indicates a compound page. Patches 1 to 6 rework the hugetlb code to allow us to reuse folio->lru for the owner ops as long as they are not isolated. Patches 7 to 10 introduce struct folio_owner_ops, and apply the callbacks to zone device and hugetlb folios. Cheers, /fuad [*] https://lore.kernel.org/all/CAGtprH_JP2w-4rq02h_Ugvq5KuHX7TUvegOS7xUs_iy5hriE7g@mail.gmail.com/ David Hildenbrand (6): mm/hugetlb: rename isolate_hugetlb() to folio_isolate_hugetlb() mm/migrate: don't call folio_putback_active_hugetlb() on dst hugetlb folio mm/hugetlb: rename "folio_putback_active_hugetlb()" to "folio_putback_hugetlb()" mm/hugetlb-cgroup: convert hugetlb_cgroup_css_offline() to work on folios mm/hugetlb: use folio->lru int demote_free_hugetlb_folios() mm/hugetlb: use separate folio->_hugetlb_list for hugetlb-internals Fuad Tabba (4): mm: Introduce struct folio_owner_ops mm: Use getters and setters to access page pgmap mm: Use owner_ops on folio_put for zone device pages mm: hugetlb: Use owner_ops on folio_put for hugetlb drivers/gpu/drm/nouveau/nouveau_dmem.c | 4 +- drivers/pci/p2pdma.c | 8 +- include/linux/hugetlb.h | 10 +- include/linux/memremap.h | 14 +- include/linux/mm_types.h | 107 ++++++++++++++- lib/test_hmm.c | 2 +- mm/gup.c | 2 +- mm/hmm.c | 2 +- mm/hugetlb.c | 179 ++++++++++++++++++------- mm/hugetlb_cgroup.c | 19 ++- mm/hugetlb_vmemmap.c | 8 +- mm/internal.h | 1 - mm/memory.c | 2 +- mm/mempolicy.c | 2 +- mm/memremap.c | 49 +------ mm/migrate.c | 20 +-- mm/migrate_device.c | 4 +- mm/mm_init.c | 48 ++++++- mm/swap.c | 25 ++-- 19 files changed, 342 insertions(+), 164 deletions(-) base-commit: beb2622b970047000fa3cae64c23585669b01fca