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Date:   Thu, 20 Feb 2020 16:12:02 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Yang Shi <shy828301@gmail.com>
Cc:     David Rientjes <rientjes@google.com>,
        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
Message-ID: <20200220131202.i77zt3zj53mimrnu@box>
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On Wed, Feb 19, 2020 at 09:01:23AM -0800, Yang Shi wrote:
> On Tue, Feb 18, 2020 at 7:44 PM David Rientjes <rientjes@google.com> wrote:
> >
> > 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.
> 
> How's about the 3rd option, adding THP_FILE_FALLBACK.

I like this option.

Problem with THP_FAULT_* is that shmem_getpage_gfp() is called not only
from fault path, but also from syscalls.

-- 
 Kirill A. Shutemov