On 30 Aug 2024, at 6:03, Usama Arif wrote:
> From: Yu Zhao <yuzhao@xxxxxxxxxx>
>
> Here being unused means containing only zeros and inaccessible to
> userspace. When splitting an isolated thp under reclaim or migration,
> the unused subpages can be mapped to the shared zeropage, hence saving
> memory. This is particularly helpful when the internal
> fragmentation of a thp is high, i.e. it has many untouched subpages.
>
> This is also a prerequisite for THP low utilization shrinker which will
> be introduced in later patches, where underutilized THPs are split, and
> the zero-filled pages are freed saving memory.
>
> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> Tested-by: Shuang Zhai <zhais@xxxxxxxxxx>
> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
> ---
> include/linux/rmap.h | 7 ++++-
> mm/huge_memory.c | 8 ++---
> mm/migrate.c | 72 ++++++++++++++++++++++++++++++++++++++------
> mm/migrate_device.c | 4 +--
> 4 files changed, 75 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 91b5935e8485..d5e93e44322e 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -745,7 +745,12 @@ int folio_mkclean(struct folio *);
> int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
> struct vm_area_struct *vma);
>
> -void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked);
> +enum rmp_flags {
> + RMP_LOCKED = 1 << 0,
> + RMP_USE_SHARED_ZEROPAGE = 1 << 1,
> +};
> +
> +void remove_migration_ptes(struct folio *src, struct folio *dst, int flags);
>
> /*
> * rmap_walk_control: To control rmap traversing for specific needs
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0c48806ccb9a..af60684e7c70 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3020,7 +3020,7 @@ bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> return false;
> }
>
> -static void remap_page(struct folio *folio, unsigned long nr)
> +static void remap_page(struct folio *folio, unsigned long nr, int flags)
> {
> int i = 0;
>
> @@ -3028,7 +3028,7 @@ static void remap_page(struct folio *folio, unsigned long nr)
> if (!folio_test_anon(folio))
> return;
> for (;;) {
> - remove_migration_ptes(folio, folio, true);
> + remove_migration_ptes(folio, folio, RMP_LOCKED | flags);
> i += folio_nr_pages(folio);
> if (i >= nr)
> break;
> @@ -3240,7 +3240,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>
> if (nr_dropped)
> shmem_uncharge(folio->mapping->host, nr_dropped);
> - remap_page(folio, nr);
> + remap_page(folio, nr, PageAnon(head) ? RMP_USE_SHARED_ZEROPAGE : 0);
>
> /*
> * set page to its compound_head when split to non order-0 pages, so
> @@ -3542,7 +3542,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> if (mapping)
> xas_unlock(&xas);
> local_irq_enable();
> - remap_page(folio, folio_nr_pages(folio));
> + remap_page(folio, folio_nr_pages(folio), 0);
> ret = -EAGAIN;
> }
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6f9c62c746be..d039863e014b 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -204,13 +204,57 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
> return true;
> }
>
> +static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> + struct folio *folio,
> + unsigned long idx)
> +{
> + struct page *page = folio_page(folio, idx);
> + bool contains_data;
> + pte_t newpte;
> + void *addr;
> +
> + VM_BUG_ON_PAGE(PageCompound(page), page);
This should be:
diff --git a/mm/migrate.c b/mm/migrate.c
index e950fd62607f..7ffdbe078aa7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -206,7 +206,8 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
pte_t newpte;
void *addr;
- VM_BUG_ON_PAGE(PageCompound(page), page);
+ if (PageCompound(page))
+ return false;
VM_BUG_ON_PAGE(!PageAnon(page), page);
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
Otherwise, splitting anonymous large folios to non order-0 ones just
triggers this BUG_ON.
> + VM_BUG_ON_PAGE(!PageAnon(page), page);
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
> +
> + if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
> + mm_forbids_zeropage(pvmw->vma->vm_mm))
> + return false;
> +
> + /*
> + * The pmd entry mapping the old thp was flushed and the pte mapping
> + * this subpage has been non present. If the subpage is only zero-filled
> + * then map it to the shared zeropage.
> + */
> + addr = kmap_local_page(page);
> + contains_data = memchr_inv(addr, 0, PAGE_SIZE);
> + kunmap_local(addr);
> +
> + if (contains_data)
> + return false;
> +
> + newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
> + pvmw->vma->vm_page_prot));
> + set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
> +
> + dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
> + return true;
> +}
> +
> +struct rmap_walk_arg {
> + struct folio *folio;
> + bool map_unused_to_zeropage;
> +};
> +
> /*
> * Restore a potential migration pte to a working pte entry
> */
> static bool remove_migration_pte(struct folio *folio,
> - struct vm_area_struct *vma, unsigned long addr, void *old)
> + struct vm_area_struct *vma, unsigned long addr, void *arg)
> {
> - DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
> + struct rmap_walk_arg *rmap_walk_arg = arg;
> + DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
>
> while (page_vma_mapped_walk(&pvmw)) {
> rmap_t rmap_flags = RMAP_NONE;
> @@ -234,6 +278,9 @@ static bool remove_migration_pte(struct folio *folio,
> continue;
> }
> #endif
> + if (rmap_walk_arg->map_unused_to_zeropage &&
> + try_to_map_unused_to_zeropage(&pvmw, folio, idx))
> + continue;
>
> folio_get(folio);
> pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
> @@ -312,14 +359,21 @@ static bool remove_migration_pte(struct folio *folio,
> * Get rid of all migration entries and replace them by
> * references to the indicated page.
> */
> -void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked)
> +void remove_migration_ptes(struct folio *src, struct folio *dst, int flags)
> {
> + struct rmap_walk_arg rmap_walk_arg = {
> + .folio = src,
> + .map_unused_to_zeropage = flags & RMP_USE_SHARED_ZEROPAGE,
> + };
> +
> struct rmap_walk_control rwc = {
> .rmap_one = remove_migration_pte,
> - .arg = src,
> + .arg = &rmap_walk_arg,
> };
>
> - if (locked)
> + VM_BUG_ON_FOLIO((flags & RMP_USE_SHARED_ZEROPAGE) && (src != dst), src);
> +
> + if (flags & RMP_LOCKED)
> rmap_walk_locked(dst, &rwc);
> else
> rmap_walk(dst, &rwc);
> @@ -934,7 +988,7 @@ static int writeout(struct address_space *mapping, struct folio *folio)
> * At this point we know that the migration attempt cannot
> * be successful.
> */
> - remove_migration_ptes(folio, folio, false);
> + remove_migration_ptes(folio, folio, 0);
>
> rc = mapping->a_ops->writepage(&folio->page, &wbc);
>
> @@ -1098,7 +1152,7 @@ static void migrate_folio_undo_src(struct folio *src,
> struct list_head *ret)
> {
> if (page_was_mapped)
> - remove_migration_ptes(src, src, false);
> + remove_migration_ptes(src, src, 0);
> /* Drop an anon_vma reference if we took one */
> if (anon_vma)
> put_anon_vma(anon_vma);
> @@ -1336,7 +1390,7 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
> lru_add_drain();
>
> if (old_page_state & PAGE_WAS_MAPPED)
> - remove_migration_ptes(src, dst, false);
> + remove_migration_ptes(src, dst, 0);
>
> out_unlock_both:
> folio_unlock(dst);
> @@ -1474,7 +1528,7 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
>
> if (page_was_mapped)
> remove_migration_ptes(src,
> - rc == MIGRATEPAGE_SUCCESS ? dst : src, false);
> + rc == MIGRATEPAGE_SUCCESS ? dst : src, 0);
>
> unlock_put_anon:
> folio_unlock(dst);
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 8d687de88a03..9cf26592ac93 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -424,7 +424,7 @@ static unsigned long migrate_device_unmap(unsigned long *src_pfns,
> continue;
>
> folio = page_folio(page);
> - remove_migration_ptes(folio, folio, false);
> + remove_migration_ptes(folio, folio, 0);
>
> src_pfns[i] = 0;
> folio_unlock(folio);
> @@ -840,7 +840,7 @@ void migrate_device_finalize(unsigned long *src_pfns,
> dst = src;
> }
>
> - remove_migration_ptes(src, dst, false);
> + remove_migration_ptes(src, dst, 0);
> folio_unlock(src);
>
> if (folio_is_zone_device(src))
> --
> 2.43.5
Best Regards,
Yan, Zi
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