Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole
From: Ryan Roberts
Date: Fri Mar 15 2024 - 06:59:33 EST
On 14/03/2024 20:43, Barry Song wrote:
> On Fri, Mar 15, 2024 at 2:57 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> 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;
>
>
> Indeed, I noticed that a couple of days ago, but it turned out that it
> didn't cause any issues
> because the condition following 'if (swp_offset(entry) != start_offset
> + i)' cannot be true :-)
>
> I do agree it needs a fix here. maybe by
>
> if (!is_swap_pte(pte))
> return false?
Nice! I hadn't noticed is_swap_pte().
>
>> 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;
>>>>>
>
> Thanks
> Barry