Re: [PATCH v2 8/9] mm/swap: introduce a helper for swapin without vmfault
From: Kairui Song
Date: Mon Jan 22 2024 - 06:42:20 EST
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.
>
> > 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.