Re: [PATCH v2 8/9] mm/swap: introduce a helper for swapin without vmfault

[Huang, Ying]

Kairui Song <ryncsn@xxxxxxxxx> writes:

> On Mon, Jan 22, 2024 at 2:40 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Kairui Song <ryncsn@xxxxxxxxx> writes:
>>
>> > On Mon, Jan 15, 2024 at 9:54 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >>
>> >> Kairui Song <ryncsn@xxxxxxxxx> writes:
>> >>
>> >> > Huang, Ying <ying.huang@xxxxxxxxx> 于2024年1月9日周二 09:11写道：
>> >> >>
>> >> >> Kairui Song <ryncsn@xxxxxxxxx> writes:
>> >> >>
>> >> >> > From: Kairui Song <kasong@xxxxxxxxxxx>
>> >> >> >
>> >> >> > There are two places where swapin is not caused by direct anon page fault:
>> >> >> > - shmem swapin, invoked indirectly through shmem mapping
>> >> >> > - swapoff
>> >> >> >
>> >> >> > They used to construct a pseudo vmfault struct for swapin function.
>> >> >> > Shmem has dropped the pseudo vmfault recently in commit ddc1a5cbc05d
>> >> >> > ("mempolicy: alloc_pages_mpol() for NUMA policy without vma"). Swapoff
>> >> >> > path is still using one.
>> >> >> >
>> >> >> > Introduce a helper for them both, this help save stack usage for swapoff
>> >> >> > path, and help apply a unified swapin cache and readahead policy check.
>> >> >> >
>> >> >> > Due to missing vmfault info, the caller have to pass in mempolicy
>> >> >> > explicitly, make it different from swapin_entry and name it
>> >> >> > swapin_entry_mpol.
>> >> >> >
>> >> >> > This commit convert swapoff to use this helper, follow-up commits will
>> >> >> > convert shmem to use it too.
>> >> >> >
>> >> >> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
>> >> >> > ---
>> >> >> >  mm/swap.h       |  9 +++++++++
>> >> >> >  mm/swap_state.c | 40 ++++++++++++++++++++++++++++++++--------
>> >> >> >  mm/swapfile.c   | 15 ++++++---------
>> >> >> >  3 files changed, 47 insertions(+), 17 deletions(-)
>> >> >> >
>> >> >> > diff --git a/mm/swap.h b/mm/swap.h
>> >> >> > index 9180411afcfe..8f790a67b948 100644
>> >> >> > --- a/mm/swap.h
>> >> >> > +++ b/mm/swap.h
>> >> >> > @@ -73,6 +73,9 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
>> >> >> >               struct mempolicy *mpol, pgoff_t ilx);
>> >> >> >  struct folio *swapin_entry(swp_entry_t entry, gfp_t flag,
>> >> >> >                           struct vm_fault *vmf, enum swap_cache_result *result);
>> >> >> > +struct folio *swapin_entry_mpol(swp_entry_t entry, gfp_t gfp_mask,
>> >> >> > +                             struct mempolicy *mpol, pgoff_t ilx,
>> >> >> > +                             enum swap_cache_result *result);
>> >> >> >
>> >> >> >  static inline unsigned int folio_swap_flags(struct folio *folio)
>> >> >> >  {
>> >> >> > @@ -109,6 +112,12 @@ static inline struct folio *swapin_entry(swp_entry_t swp, gfp_t gfp_mask,
>> >> >> >       return NULL;
>> >> >> >  }
>> >> >> >
>> >> >> > +static inline struct page *swapin_entry_mpol(swp_entry_t entry, gfp_t gfp_mask,
>> >> >> > +             struct mempolicy *mpol, pgoff_t ilx, enum swap_cache_result *result)
>> >> >> > +{
>> >> >> > +     return NULL;
>> >> >> > +}
>> >> >> > +
>> >> >> >  static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
>> >> >> >  {
>> >> >> >       return 0;
>> >> >> > diff --git a/mm/swap_state.c b/mm/swap_state.c
>> >> >> > index 21badd4f0fc7..3edf4b63158d 100644
>> >> >> > --- a/mm/swap_state.c
>> >> >> > +++ b/mm/swap_state.c
>> >> >> > @@ -880,14 +880,13 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
>> >> >> >   * in.
>> >> >> >   */
>> >> >> >  static struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
>> >> >> > -                               struct vm_fault *vmf, void *shadow)
>> >> >> > +                                struct mempolicy *mpol, pgoff_t ilx,
>> >> >> > +                                void *shadow)
>> >> >> >  {
>> >> >> > -     struct vm_area_struct *vma = vmf->vma;
>> >> >> >       struct folio *folio;
>> >> >> >
>> >> >> > -     /* skip swapcache */
>> >> >> > -     folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
>> >> >> > -                             vma, vmf->address, false);
>> >> >> > +     folio = (struct folio *)alloc_pages_mpol(gfp_mask, 0,
>> >> >> > +                     mpol, ilx, numa_node_id());
>> >> >> >       if (folio) {
>> >> >> >               if (mem_cgroup_swapin_charge_folio(folio, NULL,
>> >> >> >                                                  GFP_KERNEL, entry)) {
>> >> >> > @@ -943,18 +942,18 @@ struct folio *swapin_entry(swp_entry_t entry, gfp_t gfp_mask,
>> >> >> >               goto done;
>> >> >> >       }
>> >> >> >
>> >> >> > +     mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
>> >> >> >       if (swap_use_no_readahead(swp_swap_info(entry), entry)) {
>> >> >> > -             folio = swapin_direct(entry, gfp_mask, vmf, shadow);
>> >> >> > +             folio = swapin_direct(entry, gfp_mask, mpol, ilx, shadow);
>> >> >> >               cache_result = SWAP_CACHE_BYPASS;
>> >> >> >       } else {
>> >> >> > -             mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
>> >> >> >               if (swap_use_vma_readahead())
>> >> >> >                       folio = swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf);
>> >> >> >               else
>> >> >> >                       folio = swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
>> >> >> > -             mpol_cond_put(mpol);
>> >> >> >               cache_result = SWAP_CACHE_MISS;
>> >> >> >       }
>> >> >> > +     mpol_cond_put(mpol);
>> >> >> >  done:
>> >> >> >       if (result)
>> >> >> >               *result = cache_result;
>> >> >> > @@ -962,6 +961,31 @@ struct folio *swapin_entry(swp_entry_t entry, gfp_t gfp_mask,
>> >> >> >       return folio;
>> >> >> >  }
>> >> >> >
>> >> >> > +struct folio *swapin_entry_mpol(swp_entry_t entry, gfp_t gfp_mask,
>> >> >> > +                             struct mempolicy *mpol, pgoff_t ilx,
>> >> >> > +                             enum swap_cache_result *result)
>> >> >> > +{
>> >> >> > +     enum swap_cache_result cache_result;
>> >> >> > +     void *shadow = NULL;
>> >> >> > +     struct folio *folio;
>> >> >> > +
>> >> >> > +     folio = swap_cache_get_folio(entry, NULL, 0, &shadow);
>> >> >> > +     if (folio) {
>> >> >> > +             cache_result = SWAP_CACHE_HIT;
>> >> >> > +     } else if (swap_use_no_readahead(swp_swap_info(entry), entry)) {
>> >> >> > +             folio = swapin_direct(entry, gfp_mask, mpol, ilx, shadow);
>> >> >> > +             cache_result = SWAP_CACHE_BYPASS;
>> >> >> > +     } else {
>> >> >> > +             folio = swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
>> >> >> > +             cache_result = SWAP_CACHE_MISS;
>> >> >> > +     }
>> >> >> > +
>> >> >> > +     if (result)
>> >> >> > +             *result = cache_result;
>> >> >> > +
>> >> >> > +     return folio;
>> >> >> > +}
>> >> >> > +
>> >> >> >  #ifdef CONFIG_SYSFS
>> >> >> >  static ssize_t vma_ra_enabled_show(struct kobject *kobj,
>> >> >> >                                    struct kobj_attribute *attr, char *buf)
>> >> >> > diff --git a/mm/swapfile.c b/mm/swapfile.c
>> >> >> > index 5aa44de11edc..2f77bf143af8 100644
>> >> >> > --- a/mm/swapfile.c
>> >> >> > +++ b/mm/swapfile.c
>> >> >> > @@ -1840,18 +1840,13 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>> >> >> >       do {
>> >> >> >               struct folio *folio;
>> >> >> >               unsigned long offset;
>> >> >> > +             struct mempolicy *mpol;
>> >> >> >               unsigned char swp_count;
>> >> >> >               swp_entry_t entry;
>> >> >> > +             pgoff_t ilx;
>> >> >> >               int ret;
>> >> >> >               pte_t ptent;
>> >> >> >
>> >> >> > -             struct vm_fault vmf = {
>> >> >> > -                     .vma = vma,
>> >> >> > -                     .address = addr,
>> >> >> > -                     .real_address = addr,
>> >> >> > -                     .pmd = pmd,
>> >> >> > -             };
>> >> >> > -
>> >> >> >               if (!pte++) {
>> >> >> >                       pte = pte_offset_map(pmd, addr);
>> >> >> >                       if (!pte)
>> >> >> > @@ -1871,8 +1866,10 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>> >> >> >               pte_unmap(pte);
>> >> >> >               pte = NULL;
>> >> >> >
>> >> >> > -             folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
>> >> >> > -                                  &vmf, NULL);
>> >> >> > +             mpol = get_vma_policy(vma, addr, 0, &ilx);
>> >> >> > +             folio = swapin_entry_mpol(entry, GFP_HIGHUSER_MOVABLE,
>> >> >> > +                                       mpol, ilx, NULL);
>> >> >> > +             mpol_cond_put(mpol);
>> >> >> >               if (!folio) {
>> >> >> >                       /*
>> >> >> >                        * The entry could have been freed, and will not
>> >> >>
>> >> >> IIUC, after the change, we will always use cluster readahead for
>> >> >> swapoff.  This may be OK.  But, at least we need some test results which
>> >> >> show that this will not cause any issue for this behavior change.  And
>> >> >> the behavior change should be described explicitly in patch description.
>> >> >
>> >> > Hi Ying
>> >> >
>> >> > Actually there is a swap_use_no_readahead check in swapin_entry_mpol,
>> >> > so when readahaed is not needed (SYNC_IO), it's just skipped.
>> >> >
>> >> > And I think VMA readahead is not helpful swapoff, swapoff is already
>> >> > walking the VMA, mostly uninterrupted in kernel space. With VMA
>> >> > readahead or not, it will issue IO page by page.
>> >> > The benchmark result I posted before is actually VMA readhead vs
>> >> > no-readahead for ZRAM, sorry I didn't make it clear. It's obvious
>> >> > no-readahead is faster.
>> >> >
>> >> > For actual block device, cluster readahead might be a good choice for
>> >> > swapoff, since all pages will be read for swapoff, there has to be
>> >> > enough memory for all swapcached page to stay in memory or swapoff
>> >> > will fail anyway, and cluster read is faster for block devices.
>> >>
>> >> It's possible.  But please run the tests on some actual block devices
>> >> and show your results.  Random memory accessing pattern should be
>> >> tested, and the swap space usage should be > 50% to show some not so
>> >> friendly situation.
>> >>
>> >
>> > Hi Ying,
>> >
>> > I setup a test environment and did following test, and found that
>> > cluster readahaed for swapoff is actually much worse in default setup:
>> >
>> > 1. Setup MySQL server using 2G memcg, with 28G buffer pool, and 24G NVME swap
>> > 2. Stress test with sysbench for 15min.
>> > 3. Remove the 2G memcg limit and swapoff.
>> >
>> > Before this patch, swapoff will take about 9m.
>> > After this patch, swapoff will take about 30m.
>>
>> Thanks for data!
>>
>> > After some analysis I found the reason is that cluster readahead is
>> > almost disabled (window == 1 or 2) during swapoff, because it will
>> > detect a very low hit rate on fragmented swap. But VMA readhead is
>> > much more aggressive here since swapoff is walking the VMA, with a
>> > very high hit rate.
>> >
>> > But If I force cluster readahead to use a large window for swapoff,
>> > the swapoff performance boost by a lot:
>> > By adding following change in swap_cluster_readahead:
>> >
>> > if (unlikely(!(si->flags & SWP_WRITEOK)))
>> >     mask = max_t(unsigned long, 1 << READ_ONCE(page_cluster), PMD_SIZE
>> > / PAGE_SIZE) - 1;
>> >
>> > The swapoff will take only 40s to finish, more than 10x faster than
>> > the VMA readahead path (9m), because VMA readhead is still doing 4K
>> > random IO just with a longer queue due to async readahead. But cluster
>> > readhead will be doing 2M IO now.
>> > I think PMD size window is good here since it still keep a balance
>> > between good IO performance and the swapoff progress can still be
>> > interrupted, and the system is responsible. And in most cases we
>> > expect swapoff to success, if it fail, the RA windows should still
>> > keep the side effect of extra swapcache being generated acceptable.
>>
>> swapoff performance isn't very important because swapoff is a very rare
>> operation.  It's OK to optimize it if the change is simple and doesn't
>> compromise other stuff.  But, as you said below, using large readahead
>> window makes mempolicy issue more serious.  Why isn't the original
>> swapoff performance good enough for you?
>
> Thanks for the reply.
>
> I think I'll just keep the original VMA readahead policy here then.
> Just I noticed that VMA readhead will also violate ranged memory
> policy too... That's some different issue, looks trivial though.

During reviewing your patch, I found that too.  I think that they can be
fixed because we have enough information.

--
Best Regards,
Huang, Ying

>>
>> > But this showed a bad effect of ignoring mem policy. Actually this is
>> > not a new issue, cluster readhead is already violating VMA's mem
>> > policy since it does readhead only based on entry value not VMA, the
>> > entry being swapped in is not aware of which VMA it belongs.
>> >
>> > And I was thinking, maybe we can just drop the mpol all the way, and
>> > use the nid from page shadow to alloc pages, that may save a lot of
>> > effort, and make cluster readhead more usable in general, also might
>> > simplify a lot of code. How do you think? If this is acceptable, I
>> > think I can send out a new series first and then rework this one
>> > later.
>>
>> The "shadow" node can be reclaimed, please take a look at
>> scan_shadow_nodes().  Although this hasn't been implemented, it may be
>> implemented someday.
>
> Right, I noticed upstream commit 5649d113ffce ("swap_state: update
> shadow_nodes for anonymous page") started reclaiming anon pages
> shadows now, thanks for the info.