On Thu, Mar 06, 2025 at 03:42:35PM +1100, Balbir Singh wrote:
> When a zone device page is split (via huge pmd folio split). The
> driver callback for folio_split is invoked to let the device driver
> know that the folio size has been split into a smaller order.
>
> The HMM test driver has been updated to handle the split, since the
> test driver uses backing pages, it requires a mechanism of reorganizing
> the backing pages (backing pages are used to create a mirror device)
> again into the right sized order pages. This is supported by exporting
> prep_compound_page().
>
> Signed-off-by: Balbir Singh <balb...@nvidia.com>
> ---
> include/linux/memremap.h | 7 +++++++
> include/linux/mm.h | 1 +
> lib/test_hmm.c | 35 +++++++++++++++++++++++++++++++++++
> mm/huge_memory.c | 5 +++++
> mm/page_alloc.c | 1 +
> 5 files changed, 49 insertions(+)
>
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 11d586dd8ef1..2091b754f1da 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -100,6 +100,13 @@ struct dev_pagemap_ops {
> */
> int (*memory_failure)(struct dev_pagemap *pgmap, unsigned long pfn,
> unsigned long nr_pages, int mf_flags);
> +
> + /*
> + * Used for private (un-addressable) device memory only.
> + * This callback is used when a folio is split into
> + * a smaller folio
> + */
> + void (*folio_split)(struct folio *head, struct folio *tail);
> };
>
> #define PGMAP_ALTMAP_VALID (1 << 0)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 98a67488b5fe..3d0e91e0a923 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1415,6 +1415,7 @@ static inline struct folio *virt_to_folio(const void *x)
> void __folio_put(struct folio *folio);
>
> void split_page(struct page *page, unsigned int order);
> +void prep_compound_page(struct page *page, unsigned int order);
> void folio_copy(struct folio *dst, struct folio *src);
> int folio_mc_copy(struct folio *dst, struct folio *src);
>
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index a81d2f8a0426..18b6a7b061d7 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -1640,10 +1640,45 @@ static vm_fault_t dmirror_devmem_fault(struct
> vm_fault *vmf)
> return ret;
> }
>
> +
> +static void dmirror_devmem_folio_split(struct folio *head, struct folio
> *tail)
> +{
> + struct page *rpage = BACKING_PAGE(folio_page(head, 0));
> + struct folio *new_rfolio;
> + struct folio *rfolio;
> + unsigned long offset = 0;
> +
> + if (!rpage) {
> + folio_page(tail, 0)->zone_device_data = NULL;
> + return;
> + }
> +
> + offset = folio_pfn(tail) - folio_pfn(head);
> + rfolio = page_folio(rpage);
> + new_rfolio = page_folio(folio_page(rfolio, offset));
> +
> + folio_page(tail, 0)->zone_device_data = folio_page(new_rfolio, 0);
> +
> + if (folio_pfn(tail) - folio_pfn(head) == 1) {
> + if (folio_order(head))
> + prep_compound_page(folio_page(rfolio, 0),
> + folio_order(head));
> + folio_set_count(rfolio, 1);
> + }
> + clear_compound_head(folio_page(new_rfolio, 0));
> + if (folio_order(tail))
> + prep_compound_page(folio_page(new_rfolio, 0),
> + folio_order(tail));
> + folio_set_count(new_rfolio, 1);
> + folio_page(new_rfolio, 0)->mapping = folio_page(rfolio, 0)->mapping;
> + tail->pgmap = head->pgmap;
It seem what you're trying to do here is split a higher order driver allocted
folio (rfolio) into smaller ones right? Rather than open coding that I think it
would be benefical to create an exported helper function to do this. Pretty sure
at least DAX could also use this, as it has a need to split device folios into
smaller ones as well. That would also address my comment below.
> +}
> +
> static const struct dev_pagemap_ops dmirror_devmem_ops = {
> .page_free = dmirror_devmem_free,
> .migrate_to_ram = dmirror_devmem_fault,
> .page_free = dmirror_devmem_free,
> + .folio_split = dmirror_devmem_folio_split,
> };
>
> static int dmirror_device_init(struct dmirror_device *mdevice, int id)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 995ac8be5709..518a70d1b58a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3655,6 +3655,11 @@ static int __split_unmapped_folio(struct folio *folio,
> int new_order,
> MTHP_STAT_NR_ANON, 1);
> }
>
> + if (folio_is_device_private(origin_folio) &&
> + origin_folio->pgmap->ops->folio_split)
> + origin_folio->pgmap->ops->folio_split(
> + origin_folio, release);
> +
> /*
> * Unfreeze refcount first. Additional reference from
> * page cache.
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 17ea8fb27cbf..563f7e39aa79 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -573,6 +573,7 @@ void prep_compound_page(struct page *page, unsigned int
> order)
>
> prep_compound_head(page, order);
> }
> +EXPORT_SYMBOL_GPL(prep_compound_page);
I think this is probably too low level of a function to export, especially just
for a test.
> static inline void set_buddy_order(struct page *page, unsigned int order)
> {
> --
> 2.48.1
>
