Re: [PATCH rfc 3/4] mm: filemap: move __lruvec_stat_mod_folio() out of filemap_set_pte_range()

[Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>]

On 2024/5/8 17:33, David Hildenbrand wrote:
> On 07.05.24 15:12, Kefeng Wang wrote:
> > On 2024/5/7 19:11, David Hildenbrand wrote:
> > > On 29.04.24 09:24, Kefeng Wang wrote:
> > > > Adding __folio_add_file_rmap_ptes() which don't update lruvec stat, it
> > > > is used in filemap_set_pte_range(), with it, lruvec stat updating is
> > > > moved into the caller, no functional changes.
> > > >
> > > > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > > > ---
> > > >    include/linux/rmap.h |  2 ++
> > > >    mm/filemap.c         | 27 ++++++++++++++++++---------
> > > >    mm/rmap.c            | 16 ++++++++++++++++
> > > >    3 files changed, 36 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > > > index 7229b9baf20d..43014ddd06f9 100644
> > > > --- a/include/linux/rmap.h
> > > > +++ b/include/linux/rmap.h
> > > > @@ -242,6 +242,8 @@ void folio_add_anon_rmap_pmd(struct folio *,
> > > > struct page *,
> > > >            struct vm_area_struct *, unsigned long address, rmap_t 
> > > > flags);
> > > >    void folio_add_new_anon_rmap(struct folio *, struct 
> > > > vm_area_struct *,
> > > >            unsigned long address);
> > > > +int __folio_add_file_rmap_ptes(struct folio *, struct page *, int
> > > > nr_pages,
> > > > +        struct vm_area_struct *);
> > > >    void folio_add_file_rmap_ptes(struct folio *, struct page *, int
> > > > nr_pages,
> > > >            struct vm_area_struct *);
> > > >    #define folio_add_file_rmap_pte(folio, page, vma) \
> > > > diff --git a/mm/filemap.c b/mm/filemap.c
> > > > index 7019692daddd..3966b6616d02 100644
> > > > --- a/mm/filemap.c
> > > > +++ b/mm/filemap.c
> > > > @@ -3501,14 +3501,15 @@ static struct folio
> > > > *next_uptodate_folio(struct xa_state *xas,
> > > >    static void filemap_set_pte_range(struct vm_fault *vmf, struct folio
> > > > *folio,
> > > >                struct page *page, unsigned int nr, unsigned long addr,
> > > > -            unsigned long *rss)
> > > > +            unsigned long *rss, int *nr_mapped)
> > > >    {
> > > >        struct vm_area_struct *vma = vmf->vma;
> > > >        pte_t entry;
> > > >        entry = prepare_range_pte_entry(vmf, false, folio, page, nr, 
> > > > addr);
> > > > -    folio_add_file_rmap_ptes(folio, page, nr, vma);
> > > > +    *nr_mapped += __folio_add_file_rmap_ptes(folio, page, nr, vma);
> > > > +
> > > >        set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr);
> > > >        /* no need to invalidate: a not-present page won't be cached */
> > > > @@ -3525,7 +3526,8 @@ static void filemap_set_pte_range(struct
> > > > vm_fault *vmf, struct folio *folio,
> > > >    static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
> > > >                struct folio *folio, unsigned long start,
> > > >                unsigned long addr, unsigned int nr_pages,
> > > > -            unsigned long *rss, unsigned int *mmap_miss)
> > > > +            unsigned long *rss, int *nr_mapped,
> > > > +            unsigned int *mmap_miss)
> > > >    {
> > > >        vm_fault_t ret = 0;
> > > >        struct page *page = folio_page(folio, start);
> > > > @@ -3558,7 +3560,8 @@ static vm_fault_t filemap_map_folio_range(struct
> > > > vm_fault *vmf,
> > > >            continue;
> > > >    skip:
> > > >            if (count) {
> > > > -            filemap_set_pte_range(vmf, folio, page, count, addr, rss);
> > > > +            filemap_set_pte_range(vmf, folio, page, count, addr,
> > > > +                          rss, nr_mapped);
> > > >                if (in_range(vmf->address, addr, count * PAGE_SIZE))
> > > >                    ret = VM_FAULT_NOPAGE;
> > > >            }
> > > > @@ -3571,7 +3574,8 @@ static vm_fault_t filemap_map_folio_range(struct
> > > > vm_fault *vmf,
> > > >        } while (--nr_pages > 0);
> > > >        if (count) {
> > > > -        filemap_set_pte_range(vmf, folio, page, count, addr, rss);
> > > > +        filemap_set_pte_range(vmf, folio, page, count, addr, rss,
> > > > +                      nr_mapped);
> > > >            if (in_range(vmf->address, addr, count * PAGE_SIZE))
> > > >                ret = VM_FAULT_NOPAGE;
> > > >        }
> > > > @@ -3583,7 +3587,7 @@ static vm_fault_t filemap_map_folio_range(struct
> > > > vm_fault *vmf,
> > > >    static vm_fault_t filemap_map_order0_folio(struct vm_fault *vmf,
> > > >            struct folio *folio, unsigned long addr,
> > > > -        unsigned long *rss, unsigned int *mmap_miss)
> > > > +        unsigned long *rss, int *nr_mapped, unsigned int *mmap_miss)
> > > >    {
> > > >        vm_fault_t ret = 0;
> > > >        struct page *page = &folio->page;
> > > > @@ -3606,7 +3610,7 @@ static vm_fault_t
> > > > filemap_map_order0_folio(struct vm_fault *vmf,
> > > >        if (vmf->address == addr)
> > > >            ret = VM_FAULT_NOPAGE;
> > > > -    filemap_set_pte_range(vmf, folio, page, 1, addr, rss);
> > > > +    filemap_set_pte_range(vmf, folio, page, 1, addr, rss, nr_mapped);
> > > >        return ret;
> > > >    }
> > > > @@ -3646,6 +3650,7 @@ vm_fault_t filemap_map_pages(struct vm_fault 
> > > > *vmf,
> > > >        folio_type = mm_counter_file(folio);
> > > >        do {
> > > >            unsigned long end;
> > > > +        int nr_mapped = 0;
> > > >            addr += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
> > > >            vmf->pte += xas.xa_index - last_pgoff;
> > > > @@ -3655,11 +3660,15 @@ vm_fault_t filemap_map_pages(struct vm_fault
> > > > *vmf,
> > > >            if (!folio_test_large(folio))
> > > >                ret |= filemap_map_order0_folio(vmf,
> > > > -                    folio, addr, &rss, &mmap_miss);
> > > > +                    folio, addr, &rss, &nr_mapped,
> > > > +                    &mmap_miss);
> > > >            else
> > > >                ret |= filemap_map_folio_range(vmf, folio,
> > > >                        xas.xa_index - folio->index, addr,
> > > > -                    nr_pages, &rss, &mmap_miss);
> > > > +                    nr_pages, &rss, &nr_mapped,
> > > > +                    &mmap_miss);
> > > > +
> > > > +        __lruvec_stat_mod_folio(folio, NR_FILE_MAPPED, nr_mapped);
> > > >            folio_unlock(folio);
> > > >            folio_put(folio);
> > > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > > index 2608c40dffad..55face4024f2 100644
> > > > --- a/mm/rmap.c
> > > > +++ b/mm/rmap.c
> > > > @@ -1452,6 +1452,22 @@ static __always_inline void
> > > > __folio_add_file_rmap(struct folio *folio,
> > > >            mlock_vma_folio(folio, vma);
> > > >    }
> > > > +int __folio_add_file_rmap_ptes(struct folio *folio, struct page *page,
> > > > +        int nr_pages, struct vm_area_struct *vma)
> > > > +{
> > > > +    int nr, nr_pmdmapped = 0;
> > > > +
> > > > +    VM_WARN_ON_FOLIO(folio_test_anon(folio), folio);
> > > > +
> > > > +    nr = __folio_add_rmap(folio, page, nr_pages, RMAP_LEVEL_PTE,
> > > > +                  &nr_pmdmapped);
> > > > +
> > > > +    /* See comments in folio_add_anon_rmap_*() */
> > > > +    if (!folio_test_large(folio))
> > > > +        mlock_vma_folio(folio, vma);
> > > > +
> > > > +    return nr;
> > > > +}
> > >
> > > I'm not really a fan :/ It does make the code more complicated, and it
> > > will be harder to extend if we decide to ever account differently (e.g.,
> > > NR_SHMEM_MAPPED, additional tracking for mTHP etc).
> >
> > If more different accounts, this may lead to bad scalability.
>
> We already do it for PMD mappings.
>
> > > With large folios we'll be naturally batching already here, and I do
> >
> > Yes, it is batched with large folios，but our fs is ext4/tmpfs, there
> > are not support large folio or still upstreaming.
>
> Okay, so that will be sorted out sooner or later.
>
> > > wonder, if this is really worth for performance, or if we could find
> > > another way of batching (let the caller activate batching and drain
> > > afterwards) without exposing these details to the caller.
> >
> > It does reduce latency when batch lruvec stat updating without large
> > folio, but I can't find better way, or let's wait for the large folio
> > support on ext4/tmpfs, I also Cced memcg maintainers in patch4 to see if
> > there are any other ideas.
>
> I'm not convinced this benefit here is worth making the code more 
> complicated.
>
> Maybe we can find another way to optimize this batching in rmap code 
> without having to leak these details to the callers.
>
> For example, we could pass an optional batching structure to all rmap 
> add/rel functions that would collect these stat updates. Then we could 
> have one function to flush it and update the counters combined.
>
> Such batching could be beneficial also for page unmapping/zapping where 
> we might unmap various different folios in one go.

It sounds better and clearer, I will try it and see the results, thanks 
for your advise!

>