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Message-ID: <CAHbLzkqmS2RLAqTV6dC_W60FdwbOG2jZX0NU+zS0p2wuiK1t-Q@mail.gmail.com>
Date: Fri, 10 Jun 2022 14:24:34 -0700
From: Yang Shi <shy828301@...il.com>
To: "Zach O'Keefe" <zokeefe@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux MM <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v3 PATCH 2/7] mm: thp: introduce transhuge_vma_size_ok() helper
On Fri, Jun 10, 2022 at 9:38 AM Yang Shi <shy828301@...il.com> wrote:
>
> On Thu, Jun 9, 2022 at 5:52 PM Zach O'Keefe <zokeefe@...gle.com> wrote:
> >
> > On Thu, Jun 9, 2022 at 5:08 PM Yang Shi <shy828301@...il.com> wrote:
> > >
> > > On Thu, Jun 9, 2022 at 3:21 PM Zach O'Keefe <zokeefe@...gle.com> wrote:
> > > >
> > > > On Mon, Jun 6, 2022 at 2:44 PM Yang Shi <shy828301@...il.com> wrote:
> > > > >
> > > > > There are couple of places that check whether the vma size is ok for
> > > > > THP or not, they are open coded and duplicate, introduce
> > > > > transhuge_vma_size_ok() helper to do the job.
> > > > >
> > > > > Signed-off-by: Yang Shi <shy828301@...il.com>
> > > > > ---
> > > > > include/linux/huge_mm.h | 17 +++++++++++++++++
> > > > > mm/huge_memory.c | 5 +----
> > > > > mm/khugepaged.c | 12 ++++++------
> > > > > 3 files changed, 24 insertions(+), 10 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > > > index 648cb3ce7099..a8f61db47f2a 100644
> > > > > --- a/include/linux/huge_mm.h
> > > > > +++ b/include/linux/huge_mm.h
> > > > > @@ -116,6 +116,18 @@ extern struct kobj_attribute shmem_enabled_attr;
> > > > >
> > > > > extern unsigned long transparent_hugepage_flags;
> > > > >
> > > > > +/*
> > > > > + * The vma size has to be large enough to hold an aligned HPAGE_PMD_SIZE area.
> > > > > + */
> > > > > +static inline bool transhuge_vma_size_ok(struct vm_area_struct *vma)
> > > > > +{
> > > > > + if (round_up(vma->vm_start, HPAGE_PMD_SIZE) <
> > > > > + (vma->vm_end & HPAGE_PMD_MASK))
> > > > > + return true;
> > > > > +
> > > > > + return false;
> > > > > +}
> > > >
> > > > First time coming across round_up() - thanks for that - but for
> > > > symmetry, maybe also use round_down() for the end? No strong opinion -
> > > > just a suggestion given I've just discovered it.
> > >
> > > Yeah, round_down is fine too.
> > >
> > > >
> > > > > static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > > > > unsigned long addr)
> > > > > {
> > > > > @@ -345,6 +357,11 @@ static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
> > > > > return false;
> > > > > }
> > > > >
> > > > > +static inline bool transhuge_vma_size_ok(struct vm_area_struct *vma)
> > > > > +{
> > > > > + return false;
> > > > > +}
> > > > > +
> > > > > static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > > > > unsigned long addr)
> > > > > {
> > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > > index 48182c8fe151..36ada544e494 100644
> > > > > --- a/mm/huge_memory.c
> > > > > +++ b/mm/huge_memory.c
> > > > > @@ -71,10 +71,7 @@ unsigned long huge_zero_pfn __read_mostly = ~0UL;
> > > > >
> > > > > 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;
> > > > > -
> > > > > - if (!transhuge_vma_suitable(vma, addr))
> > > > > + if (!transhuge_vma_size_ok(vma))
> > > > > return false;
> > > > > if (vma_is_anonymous(vma))
> > > > > return __transparent_hugepage_enabled(vma);
> > > >
> > > > Do we need a check for vma->vm_pgoff alignment here, after
> > > > !vma_is_anonymous(), and now that we don't call
> > > > transhuge_vma_suitable()?
> > >
> > > Actually I was thinking about this too. But the THPeligible bit shown
> > > by smaps is a little bit ambiguous for file vma. The document says:
> > > "THPeligible" indicates whether the mapping is eligible for allocating
> > > THP pages - 1 if true, 0 otherwise.
> > >
> > > Even though it doesn't fulfill the alignment, it is still possible to
> > > get THP allocated, but just can't be PMD mapped. So the old behavior
> > > of THPeligible for file vma seems problematic, or at least doesn't
> > > match the document.
> >
> > I think the term "THP" is used ambiguously. Often, but not always, in
> > the code, folks will go out of their way to specify "hugepage-sized"
> > page vs "pmd-mapped hugepage" - but at least from my experience,
> > external documentation doesn't. Given that THP as a concept doesn't
> > make much sense without the possibility of pmd-mapping, I think
> > "THPeligible here means "pmd mappable". For example, AnonHugePages in
> > smaps means pmd-mapped anon hugepages.
>
> Yeah, depends on the expectation.
The funny thing is I was the last one who touched the THPeligible. It
seems the document needs to be updated too to make "pmd mappable" more
explicitly.
>
> >
> > That all said - the following patches will delete
> > transparent_hugepage_active() anyways.
>
> Yes, how I could forget this :-( The following removal of
> transparent_hugepage_active() will restore the old behavior.
>
> >
> > > I should elaborate this in the commit log.
> > >
> > > >
> > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > > index 84b9cf4b9be9..d0f8020164fc 100644
> > > > > --- a/mm/khugepaged.c
> > > > > +++ b/mm/khugepaged.c
> > > > > @@ -454,6 +454,9 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
> > > > > vma->vm_pgoff, HPAGE_PMD_NR))
> > > > > return false;
> > > > >
> > > > > + if (!transhuge_vma_size_ok(vma))
> > > > > + return false;
> > > > > +
> > > > > /* Enabled via shmem mount options or sysfs settings. */
> > > > > if (shmem_file(vma->vm_file))
> > > > > return shmem_huge_enabled(vma);
> > > > > @@ -512,9 +515,7 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
> > > > > unsigned long vm_flags)
> > > > > {
> > > > > if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
> > > > > - khugepaged_enabled() &&
> > > > > - (((vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK) <
> > > > > - (vma->vm_end & HPAGE_PMD_MASK))) {
> > > > > + khugepaged_enabled()) {
> > > > > if (hugepage_vma_check(vma, vm_flags))
> > > > > __khugepaged_enter(vma->vm_mm);
> > > > > }
> > > > > @@ -2142,10 +2143,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
> > > > > progress++;
> > > > > continue;
> > > > > }
> > > > > - hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> > > > > +
> > > > > + hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
> > > > > hend = vma->vm_end & HPAGE_PMD_MASK;
> > > > > - if (hstart >= hend)
> > > > > - goto skip;
> > > > > if (khugepaged_scan.address > hend)
> > > > > goto skip;
> > > > > if (khugepaged_scan.address < hstart)
> > > >
> > > > Likewise, could do round_down() here (just a suggestion)
> > >
> > > Fine to me.
> > >
> > > >
> > > > > --
> > > > > 2.26.3
> > > > >
> > > > >
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