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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