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Date:   Tue, 18 Feb 2020 19:44:10 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Yang Shi <shy828301@gmail.com>
cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Jeremy Cline <jcline@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
Subject: Re: [patch 1/2] mm, shmem: add thp fault alloc and fallback stats
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On Tue, 18 Feb 2020, Yang Shi wrote:

> > diff --git a/mm/shmem.c b/mm/shmem.c
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1502,9 +1502,8 @@ static struct page *shmem_alloc_page(gfp_t gfp,
> >         return page;
> >  }
> >
> > -static struct page *shmem_alloc_and_acct_page(gfp_t gfp,
> > -               struct inode *inode,
> > -               pgoff_t index, bool huge)
> > +static struct page *shmem_alloc_and_acct_page(gfp_t gfp, struct inode *inode,
> > +               pgoff_t index, bool fault, bool huge)
> >  {
> >         struct shmem_inode_info *info = SHMEM_I(inode);
> >         struct page *page;
> > @@ -1518,9 +1517,11 @@ static struct page *shmem_alloc_and_acct_page(gfp_t gfp,
> >         if (!shmem_inode_acct_block(inode, nr))
> >                 goto failed;
> >
> > -       if (huge)
> > +       if (huge) {
> >                 page = shmem_alloc_hugepage(gfp, info, index);
> > -       else
> > +               if (!page && fault)
> > +                       count_vm_event(THP_FAULT_FALLBACK);
> > +       } else
> >                 page = shmem_alloc_page(gfp, info, index);
> >         if (page) {
> >                 __SetPageLocked(page);
> > @@ -1832,11 +1833,10 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> >         }
> >
> >  alloc_huge:
> > -       page = shmem_alloc_and_acct_page(gfp, inode, index, true);
> > +       page = shmem_alloc_and_acct_page(gfp, inode, index, vmf, true);
> >         if (IS_ERR(page)) {
> >  alloc_nohuge:
> > -               page = shmem_alloc_and_acct_page(gfp, inode,
> > -                                                index, false);
> > +               page = shmem_alloc_and_acct_page(gfp, inode, index, vmf, false);
> >         }
> >         if (IS_ERR(page)) {
> >                 int retry = 5;
> > @@ -1871,8 +1871,11 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> >
> >         error = mem_cgroup_try_charge_delay(page, charge_mm, gfp, &memcg,
> >                                             PageTransHuge(page));
> > -       if (error)
> > +       if (error) {
> > +               if (vmf && PageTransHuge(page))
> > +                       count_vm_event(THP_FAULT_FALLBACK);
> >                 goto unacct;
> > +       }
> >         error = shmem_add_to_page_cache(page, mapping, hindex,
> >                                         NULL, gfp & GFP_RECLAIM_MASK);
> >         if (error) {
> > @@ -1883,6 +1886,8 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> >         mem_cgroup_commit_charge(page, memcg, false,
> >                                  PageTransHuge(page));
> >         lru_cache_add_anon(page);
> > +       if (vmf && PageTransHuge(page))
> > +               count_vm_event(THP_FAULT_ALLOC);
> 
> I think shmem THP alloc is accounted to THP_FILE_ALLOC. And it has
> been accounted by shmem_add_to_page_cache(). So, it sounds like a
> double count.
> 

I think we can choose to either include file allocations into both 
thp_fault_alloc and thp_fault_fallback or we can exclude them from both of 
them.  I don't think we can account for only one of them.

> >
> >         spin_lock_irq(&info->lock);
> >         info->alloced += compound_nr(page);
> >
>