On Tue, 20 Jun 2023, Jason Gunthorpe wrote: > On Tue, Jun 20, 2023 at 12:54:25PM -0700, Hugh Dickins wrote: > > On Tue, 20 Jun 2023, Jason Gunthorpe wrote: > > > On Tue, Jun 20, 2023 at 12:47:54AM -0700, Hugh Dickins wrote: > > > > Add powerpc-specific pte_free_defer(), to call pte_free() via call_rcu(). > > > > pte_free_defer() will be called inside khugepaged's retract_page_tables() > > > > loop, where allocating extra memory cannot be relied upon. This precedes > > > > the generic version to avoid build breakage from incompatible pgtable_t. > > > > > > > > This is awkward because the struct page contains only one rcu_head, but > > > > that page may be shared between PTE_FRAG_NR pagetables, each wanting to > > > > use the rcu_head at the same time: account concurrent deferrals with a > > > > heightened refcount, only the first making use of the rcu_head, but > > > > re-deferring if more deferrals arrived during its grace period. > > > > > > You didn't answer my question why we can't just move the rcu to the > > > actual free page? > > > > I thought that I had answered it, perhaps not to your satisfaction: > > > > https://lore.kernel.org/linux-mm/9130acb-193-6fdd-f8df-75766e663978@xxxxxxxxxx/ > > > > My conclusion then was: > > Not very good reasons: good enough, or can you supply a better patch? > > Oh, I guess I didn't read that email as answering the question.. > > I was saying to make pte_fragment_free() unconditionally do the > RCU. It is the only thing that uses the page->rcu_head, and it means > PPC would double RCU the final free on the TLB path, but that is > probably OK for now. This means pte_free_defer() won't do anything > special on PPC as PPC will always RCU free these things, this address > the defer concern too, I think. Overall it is easier to reason about. > > I looked at fixing the TLB stuff to avoid the double rcu but quickly > got scared that ppc was using a kmem_cache to allocate other page > table sizes so there is not a reliable struct page to get a rcu_head > from. This looks like the main challenge for ppc... We'd have to teach > the tlb code to not do its own RCU stuff for table levels that the > arch is already RCU freeing - and that won't get us to full RCU > freeing on PPC. Sorry for being so dense all along: yes, your way is unquestionably much better than mine. I guess I must have been obsessive about keeping pte_free_defer()+pte_free_now() "on the outside", as they were on x86, and never perceived how much easier it is with a small tweak inside pte_fragment_free(); and never reconsidered it since. But I'm not so keen on the double-RCU, extending this call_rcu() to all the normal cases, while still leaving the TLB batching in place: here is the replacement patch I'd prefer us to go forward with now. Many thanks! [PATCH v3 05/12] powerpc: add pte_free_defer() for pgtables sharing page Add powerpc-specific pte_free_defer(), to free table page via call_rcu(). pte_free_defer() will be called inside khugepaged's retract_page_tables() loop, where allocating extra memory cannot be relied upon. This precedes the generic version to avoid build breakage from incompatible pgtable_t. This is awkward because the struct page contains only one rcu_head, but that page may be shared between PTE_FRAG_NR pagetables, each wanting to use the rcu_head at the same time. But powerpc never reuses a fragment once it has been freed: so mark the page Active in pte_free_defer(), before calling pte_fragment_free() directly; and there call_rcu() to pte_free_now() when last fragment is freed and the page is PageActive. Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> --- arch/powerpc/include/asm/pgalloc.h | 4 ++++ arch/powerpc/mm/pgtable-frag.c | 29 ++++++++++++++++++++++++++--- 2 files changed, 30 insertions(+), 3 deletions(-) diff --git a/arch/powerpc/include/asm/pgalloc.h b/arch/powerpc/include/asm/pgalloc.h index 3360cad78ace..3a971e2a8c73 100644 --- a/arch/powerpc/include/asm/pgalloc.h +++ b/arch/powerpc/include/asm/pgalloc.h @@ -45,6 +45,10 @@ static inline void pte_free(struct mm_struct *mm, pgtable_t ptepage) pte_fragment_free((unsigned long *)ptepage, 0); } +/* arch use pte_free_defer() implementation in arch/powerpc/mm/pgtable-frag.c */ +#define pte_free_defer pte_free_defer +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable); + /* * Functions that deal with pagetables that could be at any level of * the table need to be passed an "index_size" so they know how to diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c index 20652daa1d7e..0c6b68130025 100644 --- a/arch/powerpc/mm/pgtable-frag.c +++ b/arch/powerpc/mm/pgtable-frag.c @@ -106,6 +106,15 @@ pte_t *pte_fragment_alloc(struct mm_struct *mm, int kernel) return __alloc_for_ptecache(mm, kernel); } +static void pte_free_now(struct rcu_head *head) +{ + struct page *page; + + page = container_of(head, struct page, rcu_head); + pgtable_pte_page_dtor(page); + __free_page(page); +} + void pte_fragment_free(unsigned long *table, int kernel) { struct page *page = virt_to_page(table); @@ -115,8 +124,22 @@ void pte_fragment_free(unsigned long *table, int kernel) BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0); if (atomic_dec_and_test(&page->pt_frag_refcount)) { - if (!kernel) - pgtable_pte_page_dtor(page); - __free_page(page); + if (kernel) + __free_page(page); + else if (TestClearPageActive(page)) + call_rcu(&page->rcu_head, pte_free_now); + else + pte_free_now(&page->rcu_head); } } + +#ifdef CONFIG_TRANSPARENT_HUGEPAGE +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable) +{ + struct page *page; + + page = virt_to_page(pgtable); + SetPageActive(page); + pte_fragment_free((unsigned long *)pgtable, 0); +} +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */ -- 2.35.3
