Date:   Tue, 21 Nov 2023 14:35:27 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     Chris Li <chrisl@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/24] mm/swap: move readahead policy checking into swapin_readahead

Chris Li <chrisl@...nel.org> 于2023年11月21日周二 14:18写道：
>
> On Sun, Nov 19, 2023 at 11:48 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > This makes swapin_readahead a main entry for swapin pages,
> > prepare for optimizations in later commits.
> >
> > This also makes swapoff able to make use of readahead checking
> > based on entry. Swapping off a 10G ZRAM (lzo-rle) is faster:
> >
> > Before:
> > time swapoff /dev/zram0
> > real    0m12.337s
> > user    0m0.001s
> > sys     0m12.329s
> >
> > After:
> > time swapoff /dev/zram0
> > real    0m9.728s
> > user    0m0.001s
> > sys     0m9.719s
> >
> > And what's more, because now swapoff will also make use of no-readahead
> > swapin helper, this also fixed a bug for no-readahead case (eg. ZRAM):
> > when a process that swapped out some memory previously was moved to a new
> > cgroup, and the original cgroup is dead, swapoff the swap device will
> > make the swapped in pages accounted into the process doing the swapoff
> > instead of the new cgroup the process was moved to.
> >
> > This can be easily reproduced by:
> > - Setup a ramdisk (eg. ZRAM) swap.
> > - Create memory cgroup A, B and C.
> > - Spawn process P1 in cgroup A and make it swap out some pages.
> > - Move process P1 to memory cgroup B.
> > - Destroy cgroup A.
> > - Do a swapoff in cgroup C.
> > - Swapped in pages is accounted into cgroup C.
> >
> > This patch will fix it make the swapped in pages accounted in cgroup B.
>
> Can you help me understand where the code makes this behavior change?
> As far as I can tell, the patch just allows swap off to use the code
> path to avoid read ahead and avoid swap cache path. I haven't figured
> out where the code makes the swapin charge to B.

Yes, swapoff always call swapin_readahead to swapin pages. Before this
patch, the page allocate/charge path is like this:
(Here we assume there ia only a ZRAM device so VMA readahead is used)
swapoff
  swapin_readahead
    swap_vma_readahead
      __read_swap_cache_async
        alloc_pages_mpol
        // *** Page charge happens here, and
        // note the second argument is NULL
        mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mask, entry)

After the patch:
swapoff
  swapin_readahead
    swapin_no_readahead
      vma_alloc_folio
        // *** Page charge happens here, and
        // note the second argument is vma->mm
      mem_cgroup_swapin_charge_folio(folio, vma->vm_mm, GFP_KERNEL, entry)

In the previous callpath (swap_vma_readahead), the mm struct info is
not passed to the final allocation/charge.
But now swapin_no_readahead can simply pass the mm struct to the
allocation/charge.

mem_cgroup_swapin_charge_folio will take the memcg of the mm_struct as
the charge target when the entry memcg is not online.

> Does it need to be ZRAM? Will zswap or SSD work in your example?

The "swapoff moves memory charge out of a dying cgroup" issue exists
for all swap devices, just this patch changed the behavior for ZRAM
(which bypass swapcache and uses swapin_no_readahead).

>
> >
> > The same bug exists for readahead path too, we'll fix it in later
> > commits.
>
> As discussed in another email, this behavior change is against the
> current memcg memory charging model.
> Better separate out this behavior change with code clean up.

Good suggestion.

>
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> >  mm/memory.c     | 22 +++++++---------------
> >  mm/swap.h       |  6 ++----
> >  mm/swap_state.c | 33 ++++++++++++++++++++++++++-------
> >  mm/swapfile.c   |  2 +-
> >  4 files changed, 36 insertions(+), 27 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index fba4a5229163..f4237a2e3b93 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3792,6 +3792,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >         rmap_t rmap_flags = RMAP_NONE;
> >         bool exclusive = false;
> >         swp_entry_t entry;
> > +       bool swapcached;
> >         pte_t pte;
> >         vm_fault_t ret = 0;
> >
> > @@ -3855,22 +3856,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >         swapcache = folio;
> >
> >         if (!folio) {
> > -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> > -                   __swap_count(entry) == 1) {
> > -                       /* skip swapcache and readahead */
> > -                       page = swapin_no_readahead(entry, GFP_HIGHUSER_MOVABLE,
> > -                                               vmf);
> > -                       if (page)
> > -                               folio = page_folio(page);
> > +               page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> > +                                       vmf, &swapcached);
> > +               if (page) {
> > +                       folio = page_folio(page);
> > +                       if (swapcached)
> > +                               swapcache = folio;
> >                 } else {
> > -                       page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> > -                                               vmf);
> > -                       if (page)
> > -                               folio = page_folio(page);
> > -                       swapcache = folio;
> > -               }
> > -
> > -               if (!folio) {
> >                         /*
> >                          * Back out if somebody else faulted in this pte
> >                          * while we released the pte lock.
> > diff --git a/mm/swap.h b/mm/swap.h
> > index ea4be4791394..f82d43d7b52a 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -55,9 +55,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> >  struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
> >                                     struct mempolicy *mpol, pgoff_t ilx);
> >  struct page *swapin_readahead(swp_entry_t entry, gfp_t flag,
> > -                             struct vm_fault *vmf);
> > -struct page *swapin_no_readahead(swp_entry_t entry, gfp_t flag,
> > -                                struct vm_fault *vmf);
> > +                             struct vm_fault *vmf, bool *swapcached);
> >
> >  static inline unsigned int folio_swap_flags(struct folio *folio)
> >  {
> > @@ -89,7 +87,7 @@ static inline struct page *swap_cluster_readahead(swp_entry_t entry,
> >  }
> >
> >  static inline struct page *swapin_readahead(swp_entry_t swp, gfp_t gfp_mask,
> > -                       struct vm_fault *vmf)
> > +                       struct vm_fault *vmf, bool *swapcached)
> >  {
> >         return NULL;
> >  }
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 45dd8b7c195d..fd0047ae324e 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -316,6 +316,11 @@ void free_pages_and_swap_cache(struct encoded_page **pages, int nr)
> >         release_pages(pages, nr);
> >  }
> >
> > +static inline bool swap_use_no_readahead(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > +       return data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_count(entry) == 1;
> > +}
> > +
> >  static inline bool swap_use_vma_readahead(void)
> >  {
> >         return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rotate_swap);
> > @@ -861,8 +866,8 @@ static struct page *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
> >   * Returns the struct page for entry and addr after the swap entry is read
> >   * in.
> >   */
> > -struct page *swapin_no_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > -                                struct vm_fault *vmf)
> > +static struct page *swapin_no_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > +                                       struct vm_fault *vmf)
> >  {
> >         struct vm_area_struct *vma = vmf->vma;
> >         struct page *page = NULL;
> > @@ -904,6 +909,8 @@ struct page *swapin_no_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >   * @entry: swap entry of this memory
> >   * @gfp_mask: memory allocation flags
> >   * @vmf: fault information
> > + * @swapcached: pointer to a bool used as indicator if the
> > + *              page is swapped in through swapcache.
> >   *
> >   * Returns the struct page for entry and addr, after queueing swapin.
> >   *
> > @@ -912,17 +919,29 @@ struct page *swapin_no_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >   * or vma-based(ie, virtual address based on faulty address) readahead.
> >   */
> >  struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > -                               struct vm_fault *vmf)
> > +                             struct vm_fault *vmf, bool *swapcached)
>
> At this point the function name "swapin_readahead" does not match what
> it does any more. Because readahead is just one of the behaviors it
> does. It also can do without readahead. It needs a better name. This
> function becomes a generic swapin_entry.

I renamed this function in later commits, I can rename it here to
avoid confusion.

>
> >  {
> >         struct mempolicy *mpol;
> > -       pgoff_t ilx;
> >         struct page *page;
> > +       pgoff_t ilx;
> > +       bool cached;
> >
> >         mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
> > -       page = swap_use_vma_readahead() ?
> > -               swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf) :
> > -               swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
> > +       if (swap_use_no_readahead(swp_swap_info(entry), entry)) {
> > +               page = swapin_no_readahead(entry, gfp_mask, vmf);
> > +               cached = false;
> > +       } else if (swap_use_vma_readahead()) {
> > +               page = swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf);
> > +               cached = true;
> > +       } else {
>
> Notice that which flavor of swapin_read is actually a property of the
> swap device.
> For that device, it always calls the same swapin_entry function.
>
> One idea is to have a VFS-like API for swap devices. This can be a
> swapin_entry function callback from the swap_ops struct. Difference
> swap devices just register different swapin_entry hooks. That way we
> don't need to look at the device flags to decide what to do. We can
> just call the VFS like swap_ops->swapin_entry(), the function pointer
> will point to the right implementation. Then we don't need these three
> levels if/else block. It is more flexible to register custom
> implementations of swap layouts as well. Something to consider for the
> future, you don't have to implement it in this series.

Interesting idea, we may look into this later.

>
> > +               page = swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
> > +               cached = true;
> > +       }
> >         mpol_cond_put(mpol);
> > +
> > +       if (swapcached)
> > +               *swapcached = cached;
> > +
> >         return page;
> >  }
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 756104ebd585..0142bfc71b81 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1874,7 +1874,7 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >                         };
> >
> >                         page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> > -                                               &vmf);
> > +                                               &vmf, NULL);
> >                         if (page)
> >                                 folio = page_folio(page);
> >                 }
> > --
> > 2.42.0
> >
> >

Thanks!

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