Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole

[Chuanhua Han]

Ryan Roberts <ryan.roberts@xxxxxxx> 于2024年3月13日周三 00:33写道：
>
> On 04/03/2024 08:13, Barry Song wrote:
> > From: Chuanhua Han <hanchuanhua@xxxxxxxx>
> >
> > On an embedded system like Android, more than half of anon memory is
> > actually in swap devices such as zRAM. For example, while an app is
> > switched to background, its most memory might be swapped-out.
> >
> > Now we have mTHP features, unfortunately, if we don't support large folios
> > swap-in, once those large folios are swapped-out, we immediately lose the
> > performance gain we can get through large folios and hardware optimization
> > such as CONT-PTE.
> >
> > This patch brings up mTHP swap-in support. Right now, we limit mTHP swap-in
> > to those contiguous swaps which were likely swapped out from mTHP as a
> > whole.
> >
> > Meanwhile, the current implementation only covers the SWAP_SYCHRONOUS
> > case. It doesn't support swapin_readahead as large folios yet since this
> > kind of shared memory is much less than memory mapped by single process.
> >
> > Right now, we are re-faulting large folios which are still in swapcache as a
> > whole, this can effectively decrease extra loops and early-exitings which we
> > have increased in arch_swap_restore() while supporting MTE restore for folios
> > rather than page. On the other hand, it can also decrease do_swap_page as
> > PTEs used to be set one by one even we hit a large folio in swapcache.
> >
> > Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
> > Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> >  mm/memory.c | 250 ++++++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 212 insertions(+), 38 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index e0d34d705e07..501ede745ef3 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3907,6 +3907,136 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> >       return VM_FAULT_SIGBUS;
> >  }
> >
> > +/*
> > + * check a range of PTEs are completely swap entries with
> > + * contiguous swap offsets and the same SWAP_HAS_CACHE.
> > + * pte must be first one in the range
> > + */
> > +static bool is_pte_range_contig_swap(pte_t *pte, int nr_pages)
> > +{
> > +     int i;
> > +     struct swap_info_struct *si;
> > +     swp_entry_t entry;
> > +     unsigned type;
> > +     pgoff_t start_offset;
> > +     char has_cache;
> > +
> > +     entry = pte_to_swp_entry(ptep_get_lockless(pte));
>
> Given you are getting entry locklessly, I expect it could change under you? So
> probably need to check that its a swap entry, etc. first?
The following non_swap_entry checks to see if it is a swap entry.
>
> > +     if (non_swap_entry(entry))
> > +             return false;
> > +     start_offset = swp_offset(entry);
> > +     if (start_offset % nr_pages)
> > +             return false;
> > +
> > +     si = swp_swap_info(entry);
>
> What ensures si remains valid (i.e. swapoff can't happen)? If swapoff can race,
> then swap_map may have been freed when you read it below. Holding the PTL can
> sometimes prevent it, but I don't think you're holding that here (you're using
> ptep_get_lockless(). Perhaps get_swap_device()/put_swap_device() can help?
Thank you for your review,you are righit! this place reaally needs
get_swap_device()/put_swap_device().
>
> > +     type = swp_type(entry);
> > +     has_cache = si->swap_map[start_offset] & SWAP_HAS_CACHE;
> > +     for (i = 1; i < nr_pages; i++) {
> > +             entry = pte_to_swp_entry(ptep_get_lockless(pte + i));
> > +             if (non_swap_entry(entry))
> > +                     return false;
> > +             if (swp_offset(entry) != start_offset + i)
> > +                     return false;
> > +             if (swp_type(entry) != type)
> > +                     return false;
> > +             /*
> > +              * while allocating a large folio and doing swap_read_folio for the
> > +              * SWP_SYNCHRONOUS_IO path, which is the case the being faulted pte
> > +              * doesn't have swapcache. We need to ensure all PTEs have no cache
> > +              * as well, otherwise, we might go to swap devices while the content
> > +              * is in swapcache
> > +              */
> > +             if ((si->swap_map[start_offset + i] & SWAP_HAS_CACHE) != has_cache)
> > +                     return false;
> > +     }
> > +
> > +     return true;
> > +}
>
> I created swap_pte_batch() for the swap-out series [1]. I wonder if that could
> be extended for the SWAP_HAS_CACHE checks? Possibly not because it assumes the
> PTL is held, and you are lockless here. Thought it might be of interest though.
>
> [1] https://lore.kernel.org/linux-mm/20240311150058.1122862-3-ryan.roberts@xxxxxxx/
>
Thanks. It's probably simily to ours, but as you said we are lockless
here, and we need to check has_cache.
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +/*
> > + * Get a list of all the (large) orders below PMD_ORDER that are enabled
> > + * for this vma. Then filter out the orders that can't be allocated over
> > + * the faulting address and still be fully contained in the vma.
> > + */
> > +static inline unsigned long get_alloc_folio_orders(struct vm_fault *vmf)
> > +{
> > +     struct vm_area_struct *vma = vmf->vma;
> > +     unsigned long orders;
> > +
> > +     orders = thp_vma_allowable_orders(vma, vma->vm_flags, false, true, true,
> > +                                       BIT(PMD_ORDER) - 1);
> > +     orders = thp_vma_suitable_orders(vma, vmf->address, orders);
> > +     return orders;
> > +}
> > +#endif
> > +
> > +static struct folio *alloc_swap_folio(struct vm_fault *vmf)
> > +{
> > +     struct vm_area_struct *vma = vmf->vma;
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +     unsigned long orders;
> > +     struct folio *folio;
> > +     unsigned long addr;
> > +     pte_t *pte;
> > +     gfp_t gfp;
> > +     int order;
> > +
> > +     /*
> > +      * If uffd is active for the vma we need per-page fault fidelity to
> > +      * maintain the uffd semantics.
> > +      */
> > +     if (unlikely(userfaultfd_armed(vma)))
> > +             goto fallback;
> > +
> > +     /*
> > +      * a large folio being swapped-in could be partially in
> > +      * zswap and partially in swap devices, zswap doesn't
> > +      * support large folios yet, we might get corrupted
> > +      * zero-filled data by reading all subpages from swap
> > +      * devices while some of them are actually in zswap
> > +      */
> > +     if (is_zswap_enabled())
> > +             goto fallback;
> > +
> > +     orders = get_alloc_folio_orders(vmf);
> > +     if (!orders)
> > +             goto fallback;
> > +
> > +     pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
>
> Could also briefly take PTL here, then is_pte_range_contig_swap() could be
> merged with an enhanced swap_pte_batch()?
Yes, it's easy to use a lock here, but I'm wondering if it's
necessary, because when we actually set pte in do_swap_page, we'll
hold PTL to check if the pte changes.
>
> > +     if (unlikely(!pte))
> > +             goto fallback;
> > +
> > +     /*
> > +      * For do_swap_page, find the highest order where the aligned range is
> > +      * completely swap entries with contiguous swap offsets.
> > +      */
> > +     order = highest_order(orders);
> > +     while (orders) {
> > +             addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> > +             if (is_pte_range_contig_swap(pte + pte_index(addr), 1 << order))
> > +                     break;
> > +             order = next_order(&orders, order);
> > +     }
>
> So in the common case, swap-in will pull in the same size of folio as was
> swapped-out. Is that definitely the right policy for all folio sizes? Certainly
> it makes sense for "small" large folios (e.g. up to 64K IMHO). But I'm not sure
> it makes sense for 2M THP; As the size increases the chances of actually needing
> all of the folio reduces so chances are we are wasting IO. There are similar
> arguments for CoW, where we currently copy 1 page per fault - it probably makes
> sense to copy the whole folio up to a certain size.
For 2M THP, IO overhead may not necessarily be large? :)
1.If 2M THP are continuously stored in the swap device, the IO
overhead may not be very large (such as submitting bio with one
bio_vec at a time).
2.If the process really needs this 2M data, one page-fault may perform
much better than multiple.
3.For swap devices like zram,using 2M THP might also improve
decompression efficiency.

On the other hand, if the process only needs a small part of the 2M
data (such as only frequent use of 4K page, the rest of the data is
never accessed), This is indeed give a lark to catch a kite!  :(
>
> Thanks,
> Ryan
>
> > +
> > +     pte_unmap(pte);
> > +
> > +     /* Try allocating the highest of the remaining orders. */
> > +     gfp = vma_thp_gfp_mask(vma);
> > +     while (orders) {
> > +             addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> > +             folio = vma_alloc_folio(gfp, order, vma, addr, true);
> > +             if (folio)
> > +                     return folio;
> > +             order = next_order(&orders, order);
> > +     }
> > +
> > +fallback:
> > +#endif
> > +     return vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vmf->address, false);
> > +}
> > +
> > +
> >  /*
> >   * We enter with non-exclusive mmap_lock (to exclude vma changes,
> >   * but allow concurrent faults), and pte mapped but not yet locked.
> > @@ -3928,6 +4058,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       pte_t pte;
> >       vm_fault_t ret = 0;
> >       void *shadow = NULL;
> > +     int nr_pages = 1;
> > +     unsigned long start_address;
> > +     pte_t *start_pte;
> >
> >       if (!pte_unmap_same(vmf))
> >               goto out;
> > @@ -3991,35 +4124,41 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (!folio) {
> >               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> >                   __swap_count(entry) == 1) {
> > -                     /*
> > -                      * Prevent parallel swapin from proceeding with
> > -                      * the cache flag. Otherwise, another thread may
> > -                      * finish swapin first, free the entry, and swapout
> > -                      * reusing the same entry. It's undetectable as
> > -                      * pte_same() returns true due to entry reuse.
> > -                      */
> > -                     if (swapcache_prepare(entry)) {
> > -                             /* Relax a bit to prevent rapid repeated page faults */
> > -                             schedule_timeout_uninterruptible(1);
> > -                             goto out;
> > -                     }
> > -                     need_clear_cache = true;
> > -
> >                       /* skip swapcache */
> > -                     folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> > -                                             vma, vmf->address, false);
> > +                     folio = alloc_swap_folio(vmf);
> >                       page = &folio->page;
> >                       if (folio) {
> >                               __folio_set_locked(folio);
> >                               __folio_set_swapbacked(folio);
> >
> > +                             if (folio_test_large(folio)) {
> > +                                     nr_pages = folio_nr_pages(folio);
> > +                                     entry.val = ALIGN_DOWN(entry.val, nr_pages);
> > +                             }
> > +
> > +                             /*
> > +                              * Prevent parallel swapin from proceeding with
> > +                              * the cache flag. Otherwise, another thread may
> > +                              * finish swapin first, free the entry, and swapout
> > +                              * reusing the same entry. It's undetectable as
> > +                              * pte_same() returns true due to entry reuse.
> > +                              */
> > +                             if (swapcache_prepare_nr(entry, nr_pages)) {
> > +                                     /* Relax a bit to prevent rapid repeated page faults */
> > +                                     schedule_timeout_uninterruptible(1);
> > +                                     goto out;
> > +                             }
> > +                             need_clear_cache = true;
> > +
> >                               if (mem_cgroup_swapin_charge_folio(folio,
> >                                                       vma->vm_mm, GFP_KERNEL,
> >                                                       entry)) {
> >                                       ret = VM_FAULT_OOM;
> >                                       goto out_page;
> >                               }
> > -                             mem_cgroup_swapin_uncharge_swap(entry);
> > +
> > +                             for (swp_entry_t e = entry; e.val < entry.val + nr_pages; e.val++)
> > +                                     mem_cgroup_swapin_uncharge_swap(e);
> >
> >                               shadow = get_shadow_from_swap_cache(entry);
> >                               if (shadow)
> > @@ -4118,6 +4257,42 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >        */
> >       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> >                       &vmf->ptl);
> > +
> > +     start_address = vmf->address;
> > +     start_pte = vmf->pte;
> > +     if (start_pte && folio_test_large(folio)) {
> > +             unsigned long nr = folio_nr_pages(folio);
> > +             unsigned long addr = ALIGN_DOWN(vmf->address, nr * PAGE_SIZE);
> > +             pte_t *aligned_pte = vmf->pte - (vmf->address - addr) / PAGE_SIZE;
> > +
> > +             /*
> > +              * case 1: we are allocating large_folio, try to map it as a whole
> > +              * iff the swap entries are still entirely mapped;
> > +              * case 2: we hit a large folio in swapcache, and all swap entries
> > +              * are still entirely mapped, try to map a large folio as a whole.
> > +              * otherwise, map only the faulting page within the large folio
> > +              * which is swapcache
> > +              */
> > +             if (!is_pte_range_contig_swap(aligned_pte, nr)) {
> > +                     if (nr_pages > 1) /* ptes have changed for case 1 */
> > +                             goto out_nomap;
> > +                     goto check_pte;
> > +             }
> > +
> > +             start_address = addr;
> > +             start_pte = aligned_pte;
> > +             /*
> > +              * the below has been done before swap_read_folio()
> > +              * for case 1
> > +              */
> > +             if (unlikely(folio == swapcache)) {
> > +                     nr_pages = nr;
> > +                     entry.val = ALIGN_DOWN(entry.val, nr_pages);
> > +                     page = &folio->page;
> > +             }
> > +     }
> > +
> > +check_pte:
> >       if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
> >               goto out_nomap;
> >
> > @@ -4185,12 +4360,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >        * We're already holding a reference on the page but haven't mapped it
> >        * yet.
> >        */
> > -     swap_free(entry);
> > +     swap_nr_free(entry, nr_pages);
> >       if (should_try_to_free_swap(folio, vma, vmf->flags))
> >               folio_free_swap(folio);
> >
> > -     inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> > -     dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> > +     folio_ref_add(folio, nr_pages - 1);
> > +     add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> > +     add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
> > +
> >       pte = mk_pte(page, vma->vm_page_prot);
> >
> >       /*
> > @@ -4200,14 +4377,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >        * exclusivity.
> >        */
> >       if (!folio_test_ksm(folio) &&
> > -         (exclusive || folio_ref_count(folio) == 1)) {
> > +         (exclusive || folio_ref_count(folio) == nr_pages)) {
> >               if (vmf->flags & FAULT_FLAG_WRITE) {
> >                       pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> >                       vmf->flags &= ~FAULT_FLAG_WRITE;
> >               }
> >               rmap_flags |= RMAP_EXCLUSIVE;
> >       }
> > -     flush_icache_page(vma, page);
> > +     flush_icache_pages(vma, page, nr_pages);
> >       if (pte_swp_soft_dirty(vmf->orig_pte))
> >               pte = pte_mksoft_dirty(pte);
> >       if (pte_swp_uffd_wp(vmf->orig_pte))
> > @@ -4216,17 +4393,19 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >
> >       /* ksm created a completely new copy */
> >       if (unlikely(folio != swapcache && swapcache)) {
> > -             folio_add_new_anon_rmap(folio, vma, vmf->address);
> > +             folio_add_new_anon_rmap(folio, vma, start_address);
> >               folio_add_lru_vma(folio, vma);
> > +     } else if (!folio_test_anon(folio)) {
> > +             folio_add_new_anon_rmap(folio, vma, start_address);
> >       } else {
> > -             folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
> > +             folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, start_address,
> >                                       rmap_flags);
> >       }
> >
> >       VM_BUG_ON(!folio_test_anon(folio) ||
> >                       (pte_write(pte) && !PageAnonExclusive(page)));
> > -     set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> > -     arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> > +     set_ptes(vma->vm_mm, start_address, start_pte, pte, nr_pages);
> > +     arch_do_swap_page(vma->vm_mm, vma, start_address, pte, vmf->orig_pte);
> >
> >       folio_unlock(folio);
> >       if (folio != swapcache && swapcache) {
> > @@ -4243,6 +4422,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       }
> >
> >       if (vmf->flags & FAULT_FLAG_WRITE) {
> > +             if (nr_pages > 1)
> > +                     vmf->orig_pte = ptep_get(vmf->pte);
> > +
> >               ret |= do_wp_page(vmf);
> >               if (ret & VM_FAULT_ERROR)
> >                       ret &= VM_FAULT_ERROR;
> > @@ -4250,14 +4432,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       }
> >
> >       /* No need to invalidate - it was non-present before */
> > -     update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> > +     update_mmu_cache_range(vmf, vma, start_address, start_pte, nr_pages);
> >  unlock:
> >       if (vmf->pte)
> >               pte_unmap_unlock(vmf->pte, vmf->ptl);
> >  out:
> >       /* Clear the swap cache pin for direct swapin after PTL unlock */
> >       if (need_clear_cache)
> > -             swapcache_clear(si, entry);
> > +             swapcache_clear_nr(si, entry, nr_pages);
> >       if (si)
> >               put_swap_device(si);
> >       return ret;
> > @@ -4273,7 +4455,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >               folio_put(swapcache);
> >       }
> >       if (need_clear_cache)
> > -             swapcache_clear(si, entry);
> > +             swapcache_clear_nr(si, entry, nr_pages);
> >       if (si)
> >               put_swap_device(si);
> >       return ret;
> > @@ -4309,15 +4491,7 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> >       if (unlikely(userfaultfd_armed(vma)))
> >               goto fallback;
> >
> > -     /*
> > -      * Get a list of all the (large) orders below PMD_ORDER that are enabled
> > -      * for this vma. Then filter out the orders that can't be allocated over
> > -      * the faulting address and still be fully contained in the vma.
> > -      */
> > -     orders = thp_vma_allowable_orders(vma, vma->vm_flags, false, true, true,
> > -                                       BIT(PMD_ORDER) - 1);
> > -     orders = thp_vma_suitable_orders(vma, vmf->address, orders);
> > -
> > +     orders = get_alloc_folio_orders(vmf);
> >       if (!orders)
> >               goto fallback;
> >
>
>


-- 
Thanks,
Chuanhua