Message-ID: <CAHbLzkqtOa61w4CQEXPsVa_JiUvMji9PdVF1BA_XW6LJ4RA_iw@mail.gmail.com>
Date:   Thu, 24 Feb 2022 13:56:37 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zi Yan <ziy@...dia.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Song Liu <songliubraving@...com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Rik van Riel <riel@...riel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Minchan Kim <minchan@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4] mm/huge_memory.c: add missing read-only THP checking
 in transparent_hugepage_enabled()

On Thu, Feb 24, 2022 at 1:29 PM Yang Shi <shy828301@...il.com> wrote:
>
> On Thu, Feb 24, 2022 at 10:51 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >
> > On 5/14/21 11:30, Miaohe Lin wrote:
> > > Since commit 99cb0dbd47a1 ("mm,thp: add read-only THP support for
> > > (non-shmem) FS"), read-only THP file mapping is supported. But it
> > > forgot to add checking for it in transparent_hugepage_enabled().
> > > To fix it, we add checking for read-only THP file mapping and also
> > > introduce helper transhuge_vma_enabled() to check whether thp is
> > > enabled for specified vma to reduce duplicated code. We rename
> > > transparent_hugepage_enabled to transparent_hugepage_active to make
> > > the code easier to follow as suggested by David Hildenbrand.
> > >
> > > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> > > Reviewed-by: Yang Shi <shy828301@...il.com>
> > > Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> >
> > FYI, I stumbled upon needing this for my reproducer for the read()
> > corruption [1] to work, and I think it's still not ideal. I also needed
> > madvise(MADV_HUGEPAGE) for the executable file mapping to make sure
> > khugepaged acts on my process. Yeah, commit 99cb0dbd47a1 suggests madvise.
> > Yet, khugepaged will happily collapse file mappings even without madvise
> > anyway. However, it might not know about the mm at all unless
> > __khugepaged_enter() has been applied at least on one of its vma's.
> > madvise() is one way to do it for executable file mappings, but it can also
> > happen through e.g. do_huge_pmd_anonymous_page() on another mapping, which
> > has nothing to do with the file mapping.
> > So what I'm trying to say is that we are somewhat inconsistent - the rules
> > to consider a vma in khugepaged seem to be clear and result of admin
> > configuration and madvise, but the rules to consider a mm for khugepaged
> > (which is the only way to collapse file mappings) are not completely
> > identical and there might be "random luck" involved.
>
> Yes, khugepaged_enter() is not called for file vma explicitly. My wild
> guess is THP for readonly fs was assumed to be collapsed by explicit
> demand from the users, for example, madvise.
>
> To achieve more consistent behavior we could call khugepaged_enter()
> for file fault just like what huge anonymous fault does. Of course we
> need to fix khugepaged_enter() to do the proper check as well.

Or we could follow shmem's implementation to call khugepaged_enter()
in .mmap method for each filesystem so that khugepaged_enter() doesn't
have to be called for each filemap_fault(). Luckily most filesystems
use generic_file_mmap().

>
> >
> > [1] https://lore.kernel.org/all/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/
> >
> > > ---
> > > v3->v4:
> > >   collect Reviewed-by tag
> > >   define transhuge_vma_enabled next to transhuge_vma_suitable
> > > ---
> > >  fs/proc/task_mmu.c      |  2 +-
> > >  include/linux/huge_mm.h | 57 +++++++++++++++++++++++++----------------
> > >  mm/huge_memory.c        | 11 +++++++-
> > >  mm/khugepaged.c         |  4 +--
> > >  mm/shmem.c              |  3 +--
> > >  5 files changed, 48 insertions(+), 29 deletions(-)
> > >
> > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > index fc9784544b24..7389df326edd 100644
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -832,7 +832,7 @@ static int show_smap(struct seq_file *m, void *v)
> > >       __show_smap(m, &mss, false);
> > >
> > >       seq_printf(m, "THPeligible:    %d\n",
> > > -                transparent_hugepage_enabled(vma));
> > > +                transparent_hugepage_active(vma));
> > >
> > >       if (arch_pkeys_enabled())
> > >               seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
> > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > index 0a526f211fec..7b7f7b52ccb8 100644
> > > --- a/include/linux/huge_mm.h
> > > +++ b/include/linux/huge_mm.h
> > > @@ -115,9 +115,34 @@ extern struct kobj_attribute shmem_enabled_attr;
> > >
> > >  extern unsigned long transparent_hugepage_flags;
> > >
> > > +static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > > +             unsigned long haddr)
> > > +{
> > > +     /* Don't have to check pgoff for anonymous vma */
> > > +     if (!vma_is_anonymous(vma)) {
> > > +             if (!IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> > > +                             HPAGE_PMD_NR))
> > > +                     return false;
> > > +     }
> > > +
> > > +     if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
> > > +             return false;
> > > +     return true;
> > > +}
> > > +
> > > +static inline bool transhuge_vma_enabled(struct vm_area_struct *vma,
> > > +                                       unsigned long vm_flags)
> > > +{
> > > +     /* Explicitly disabled through madvise. */
> > > +     if ((vm_flags & VM_NOHUGEPAGE) ||
> > > +         test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> > > +             return false;
> > > +     return true;
> > > +}
> > > +
> > >  /*
> > >   * to be used on vmas which are known to support THP.
> > > - * Use transparent_hugepage_enabled otherwise
> > > + * Use transparent_hugepage_active otherwise
> > >   */
> > >  static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
> > >  {
> > > @@ -128,15 +153,12 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
> > >       if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_NEVER_DAX))
> > >               return false;
> > >
> > > -     if (vma->vm_flags & VM_NOHUGEPAGE)
> > > +     if (!transhuge_vma_enabled(vma, vma->vm_flags))
> > >               return false;
> > >
> > >       if (vma_is_temporary_stack(vma))
> > >               return false;
> > >
> > > -     if (test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> > > -             return false;
> > > -
> > >       if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_FLAG))
> > >               return true;
> > >
> > > @@ -150,22 +172,7 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
> > >       return false;
> > >  }
> > >
> > > -bool transparent_hugepage_enabled(struct vm_area_struct *vma);
> > > -
> > > -static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > > -             unsigned long haddr)
> > > -{
> > > -     /* Don't have to check pgoff for anonymous vma */
> > > -     if (!vma_is_anonymous(vma)) {
> > > -             if (!IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> > > -                             HPAGE_PMD_NR))
> > > -                     return false;
> > > -     }
> > > -
> > > -     if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
> > > -             return false;
> > > -     return true;
> > > -}
> > > +bool transparent_hugepage_active(struct vm_area_struct *vma);
> > >
> > >  #define transparent_hugepage_use_zero_page()                         \
> > >       (transparent_hugepage_flags &                                   \
> > > @@ -351,7 +358,7 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
> > >       return false;
> > >  }
> > >
> > > -static inline bool transparent_hugepage_enabled(struct vm_area_struct *vma)
> > > +static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
> > >  {
> > >       return false;
> > >  }
> > > @@ -362,6 +369,12 @@ static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > >       return false;
> > >  }
> > >
> > > +static inline bool transhuge_vma_enabled(struct vm_area_struct *vma,
> > > +                                       unsigned long vm_flags)
> > > +{
> > > +     return false;
> > > +}
> > > +
> > >  static inline void prep_transhuge_page(struct page *page) {}
> > >
> > >  static inline bool is_transparent_hugepage(struct page *page)
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 76ca1eb2a223..4f37867eed12 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -63,7 +63,14 @@ static struct shrinker deferred_split_shrinker;
> > >  static atomic_t huge_zero_refcount;
> > >  struct page *huge_zero_page __read_mostly;
> > >
> > > -bool transparent_hugepage_enabled(struct vm_area_struct *vma)
> > > +static inline bool file_thp_enabled(struct vm_area_struct *vma)
> > > +{
> > > +     return transhuge_vma_enabled(vma, vma->vm_flags) && vma->vm_file &&
> > > +            !inode_is_open_for_write(vma->vm_file->f_inode) &&
> > > +            (vma->vm_flags & VM_EXEC);
> > > +}
> > > +
> > > +bool transparent_hugepage_active(struct vm_area_struct *vma)
> > >  {
> > >       /* The addr is used to check if the vma size fits */
> > >       unsigned long addr = (vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE;
> > > @@ -74,6 +81,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
> > >               return __transparent_hugepage_enabled(vma);
> > >       if (vma_is_shmem(vma))
> > >               return shmem_huge_enabled(vma);
> > > +     if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS))
> > > +             return file_thp_enabled(vma);
> > >
> > >       return false;
> > >  }
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 6c0185fdd815..d97b20fad6e8 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -442,9 +442,7 @@ static inline int khugepaged_test_exit(struct mm_struct *mm)
> > >  static bool hugepage_vma_check(struct vm_area_struct *vma,
> > >                              unsigned long vm_flags)
> > >  {
> > > -     /* Explicitly disabled through madvise. */
> > > -     if ((vm_flags & VM_NOHUGEPAGE) ||
> > > -         test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> > > +     if (!transhuge_vma_enabled(vma, vm_flags))
> > >               return false;
> > >
> > >       /* Enabled via shmem mount options or sysfs settings. */
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index a08cedefbfaa..1dcbec313c70 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -4032,8 +4032,7 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
> > >       loff_t i_size;
> > >       pgoff_t off;
> > >
> > > -     if ((vma->vm_flags & VM_NOHUGEPAGE) ||
> > > -         test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> > > +     if (!transhuge_vma_enabled(vma, vma->vm_flags))
> > >               return false;
> > >       if (shmem_huge == SHMEM_HUGE_FORCE)
> > >               return true;
> >

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