Received-SPF: pass (google.com: domain of linux-kernel+bounces-198825-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Precedence: bulk
MIME-Version: 1.0
References: <cover.1717033868.git.baolin.wang@linux.alibaba.com>
 <bf80d4a792ea82ab066f819ad7d10ed22a2f8e66.1717033868.git.baolin.wang@linux.alibaba.com>
 <CAGsJ_4z60mrjuQ5qKCKn0+knk_M1dy=NsH4nVLqe5Khue_5gFw@mail.gmail.com>
 <e05df24a-6254-430e-88ca-6db23e5c6bab@linux.alibaba.com> <CAGsJ_4zsR=xA4sCSyMc0OafaRdp7ibRGpdc00mDFEqT6pSQbKA@mail.gmail.com>
In-Reply-To: <CAGsJ_4zsR=xA4sCSyMc0OafaRdp7ibRGpdc00mDFEqT6pSQbKA@mail.gmail.com>
From: Barry Song <21cnbao@gmail.com>
Date: Mon, 3 Jun 2024 21:01:24 +1200
Message-ID: <CAGsJ_4yKaag+ryGvS=JMoYBXaKRTUoMacNXxgUCGe4YjucdqbQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/6] mm: memory: extend finish_fault() to support large folio
To: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: akpm@linux-foundation.org, hughd@google.com, willy@infradead.org, 
	david@redhat.com, wangkefeng.wang@huawei.com, ying.huang@intel.com, 
	ryan.roberts@arm.com, shy828301@gmail.com, ziy@nvidia.com, 
	ioworker0@gmail.com, da.gomez@samsung.com, p.raghav@samsung.com, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Mon, Jun 3, 2024 at 8:58=E2=80=AFPM Barry Song <21cnbao@gmail.com> wrote=
:
>
> On Mon, Jun 3, 2024 at 8:29=E2=80=AFPM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
> >
> >
> >
> > On 2024/6/3 13:28, Barry Song wrote:
> > > On Thu, May 30, 2024 at 2:04=E2=80=AFPM Baolin Wang
> > > <baolin.wang@linux.alibaba.com> wrote:
> > >>
> > >> Add large folio mapping establishment support for finish_fault() as =
a preparation,
> > >> to support multi-size THP allocation of anonymous shmem pages in the=
 following
> > >> patches.
> > >>
> > >> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > >> ---
> > >>   mm/memory.c | 58 ++++++++++++++++++++++++++++++++++++++++++++-----=
----
> > >>   1 file changed, 48 insertions(+), 10 deletions(-)
> > >>
> > >> diff --git a/mm/memory.c b/mm/memory.c
> > >> index eef4e482c0c2..435187ff7ea4 100644
> > >> --- a/mm/memory.c
> > >> +++ b/mm/memory.c
> > >> @@ -4831,9 +4831,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> > >>   {
> > >>          struct vm_area_struct *vma =3D vmf->vma;
> > >>          struct page *page;
> > >> +       struct folio *folio;
> > >>          vm_fault_t ret;
> > >>          bool is_cow =3D (vmf->flags & FAULT_FLAG_WRITE) &&
> > >>                        !(vma->vm_flags & VM_SHARED);
> > >> +       int type, nr_pages, i;
> > >> +       unsigned long addr =3D vmf->address;
> > >>
> > >>          /* Did we COW the page? */
> > >>          if (is_cow)
> > >> @@ -4864,24 +4867,59 @@ vm_fault_t finish_fault(struct vm_fault *vmf=
)
> > >>                          return VM_FAULT_OOM;
> > >>          }
> > >>
> > >> +       folio =3D page_folio(page);
> > >> +       nr_pages =3D folio_nr_pages(folio);
> > >> +
> > >> +       /*
> > >> +        * Using per-page fault to maintain the uffd semantics, and =
same
> > >> +        * approach also applies to non-anonymous-shmem faults to av=
oid
> > >> +        * inflating the RSS of the process.
> > >
> > > I don't feel the comment explains the root cause.
> > > For non-shmem, anyway we have allocated the memory? Avoiding inflatin=
g
> > > RSS seems not so useful as we have occupied the memory. the memory fo=
otprint
> >
> > This is also to keep the same behavior as before for non-anon-shmem, an=
d
> > will be discussed in the future.
>
> OK.
>
> >
> > > is what we really care about. so we want to rely on read-ahead hints =
of subpage
> > > to determine read-ahead size? that is why we don't map nr_pages for n=
on-shmem
> > > files though we can potentially reduce nr_pages - 1 page faults?
> >
> > IMHO, there is 2 cases for non-anon-shmem:
> > (1) read mmap() faults: we can rely on the 'fault_around_bytes'
> > interface to determin what size of mapping to build.
> > (2) writable mmap() faults: I want to keep the same behavior as before
> > (per-page fault), but we can talk about this when I send new patches to
> > use mTHP to control large folio allocation for writable mmap().
>
> OK.
>
> >
> > >> +        */
> > >> +       if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vm=
a))) {
> > >> +               nr_pages =3D 1;
> > >> +       } else if (nr_pages > 1) {
> > >> +               pgoff_t idx =3D folio_page_idx(folio, page);
> > >> +               /* The page offset of vmf->address within the VMA. *=
/
> > >> +               pgoff_t vma_off =3D vmf->pgoff - vmf->vma->vm_pgoff;
> > >> +
> > >> +               /*
> > >> +                * Fallback to per-page fault in case the folio size=
 in page
> > >> +                * cache beyond the VMA limits.
> > >> +                */
> > >> +               if (unlikely(vma_off < idx ||
> > >> +                            vma_off + (nr_pages - idx) > vma_pages(=
vma))) {
> > >> +                       nr_pages =3D 1;
> > >> +               } else {
> > >> +                       /* Now we can set mappings for the whole lar=
ge folio. */
> > >> +                       addr =3D vmf->address - idx * PAGE_SIZE;
> > >> +                       page =3D &folio->page;
> > >> +               }
> > >> +       }
> > >> +
> > >>          vmf->pte =3D pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> > >> -                                     vmf->address, &vmf->ptl);
> > >> +                                      addr, &vmf->ptl);
> > >>          if (!vmf->pte)
> > >>                  return VM_FAULT_NOPAGE;
> > >>
> > >>          /* Re-check under ptl */
> > >> -       if (likely(!vmf_pte_changed(vmf))) {
> > >> -               struct folio *folio =3D page_folio(page);
> > >> -               int type =3D is_cow ? MM_ANONPAGES : mm_counter_file=
(folio);
> > >> -
> > >> -               set_pte_range(vmf, folio, page, 1, vmf->address);
> > >> -               add_mm_counter(vma->vm_mm, type, 1);
> > >> -               ret =3D 0;
> > >> -       } else {
> > >> -               update_mmu_tlb(vma, vmf->address, vmf->pte);
> > >> +       if (nr_pages =3D=3D 1 && unlikely(vmf_pte_changed(vmf))) {
> > >> +               update_mmu_tlb(vma, addr, vmf->pte);
> > >>                  ret =3D VM_FAULT_NOPAGE;
> > >> +               goto unlock;
> > >> +       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_page=
s)) {
> > >
> > > In what case we can't use !pte_range_none(vmf->pte, 1) for nr_pages =
=3D=3D 1
> > > then unify the code for nr_pages=3D=3D1 and nr_pages > 1?
> > >
> > > It seems this has been discussed before, but I forget the reason.
> >
> > IIUC, this is for uffd case, which is not a none pte entry.
>
> Is it possible to have a COW case for shmem? For example, if someone
> maps a shmem
> file as read-only and then writes to it, would that prevent the use of
> pte_range_none?

sorry, i mean PRIVATE but not READ-ONLY.

>
> Furthermore, if we encounter a large folio in shmem while reading,
> does it necessarily
> mean we can map the entire folio? Is it possible for some processes to
> only map part
> of large folios? For instance, if process A allocates large folios and
> process B maps
> only part of this shmem file or partially unmaps a large folio, how
> would that be handled?
>
> Apologies for not debugging this thoroughly, but these two corner
> cases seem worth
> considering. If these scenarios have already been addressed, please disre=
gard my
> comments.
>
> Thanks
> Barry