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

[Ryan Roberts]

On 14/03/2024 12:56, Chuanhua Han wrote:
> 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.

No, it checks if something already known to be a "swap entry" type is actually
describing a swap entry, or a non-swap entry (e.g. migration entry, hwpoison
entry, etc.) Swap entries with type >= MAX_SWAPFILES don't actually describe swap:

static inline int non_swap_entry(swp_entry_t entry)
{
	return swp_type(entry) >= MAX_SWAPFILES;
}


So you need to do something like:

pte = ptep_get_lockless(pte);
if (pte_none(pte) || !pte_present(pte))
	return false;
entry = pte_to_swp_entry(pte);
if (non_swap_entry(entry))
	return false;
..

>>
>>> +     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!  :(

Yes indeed. It's not always clear-cut what the best thing to do is. It would be
good to hear from others on this.

>>
>> 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;
>>>
>>
>>
> 
>