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From:   SeongJae Park <sjpark@amazon.com>
To:     Shakeel Butt <shakeelb@google.com>
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Subject: Re: [PATCH v23 05/15] mm/damon: Implement primitives for the virtual memory address spaces
Date:   Wed, 23 Dec 2020 17:47:00 +0100
Message-ID: <20201223164700.29723-1-sjpark@amazon.com>
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On Wed, 23 Dec 2020 07:31:45 -0800 Shakeel Butt <shakeelb@google.com> wrote:

> On Tue, Dec 15, 2020 at 3:58 AM SeongJae Park <sjpark@amazon.com> wrote:
> >
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > This commit introduces a reference implementation of the address space
> > specific low level primitives for the virtual address space, so that
> > users of DAMON can easily monitor the data accesses on virtual address
> > spaces of specific processes by simply configuring the implementation to
> > be used by DAMON.
> >
> > The low level primitives for the fundamental access monitoring are
> > defined in two parts:
> >
> > 1. Identification of the monitoring target address range for the address
> >    space.
> > 2. Access check of specific address range in the target space.
> >
> > The reference implementation for the virtual address space does the
> > works as below.
> >
> > PTE Accessed-bit Based Access Check
> > -----------------------------------
> >
> > The implementation uses PTE Accessed-bit for basic access checks.  That
> > is, it clears the bit for next sampling target page and checks whether
> 
> 'for the next'
> 
> > it set again after one sampling period.  This could disturb the reclaim
> 
> 'it is set'

Good catch!  Will fix in the next version.

> 
> > logic.  DAMON uses ``PG_idle`` and ``PG_young`` page flags to solve the
> > conflict, as Idle page tracking does.
> >
> > VMA-based Target Address Range Construction
> > -------------------------------------------
> >
> > Only small parts in the super-huge virtual address space of the
> > processes are mapped to physical memory and accessed.  Thus, tracking
> > the unmapped address regions is just wasteful.  However, because DAMON
> > can deal with some level of noise using the adaptive regions adjustment
> > mechanism, tracking every mapping is not strictly required but could
> > even incur a high overhead in some cases.  That said, too huge unmapped
> > areas inside the monitoring target should be removed to not take the
> > time for the adaptive mechanism.
> >
> > For the reason, this implementation converts the complex mappings to
> > three distinct regions that cover every mapped area of the address
> > space.  Also, the two gaps between the three regions are the two biggest
> > unmapped areas in the given address space.  The two biggest unmapped
> > areas would be the gap between the heap and the uppermost mmap()-ed
> > region, and the gap between the lowermost mmap()-ed region and the stack
> > in most of the cases.  Because these gaps are exceptionally huge in
> > usual address spacees, excluding these will be sufficient to make a
> 
> *spaces

Good eye!  Will fix in the next version.

> 
> > reasonable trade-off.  Below shows this in detail::
> >
> >     <heap>
> >     <BIG UNMAPPED REGION 1>
> >     <uppermost mmap()-ed region>
> >     (small mmap()-ed regions and munmap()-ed regions)
> >     <lowermost mmap()-ed region>
> >     <BIG UNMAPPED REGION 2>
> >     <stack>
> >
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > Reviewed-by: Leonard Foerster <foersleo@amazon.de>
> > ---
> >  include/linux/damon.h |  13 +
> >  mm/damon/Kconfig      |   9 +
> >  mm/damon/Makefile     |   1 +
> >  mm/damon/vaddr.c      | 579 ++++++++++++++++++++++++++++++++++++++++++
> >  4 files changed, 602 insertions(+)
> >  create mode 100644 mm/damon/vaddr.c
> >
> > diff --git a/include/linux/damon.h b/include/linux/damon.h
> > index f446f8433599..39b4d6d3ddee 100644
> > --- a/include/linux/damon.h
> > +++ b/include/linux/damon.h
> > @@ -274,4 +274,17 @@ int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
> >
> >  #endif /* CONFIG_DAMON */
> >
> > +#ifdef CONFIG_DAMON_VADDR
> > +
> > +/* Monitoring primitives for virtual memory address spaces */
> > +void damon_va_init_regions(struct damon_ctx *ctx);
> > +void damon_va_update_regions(struct damon_ctx *ctx);
> > +void damon_va_prepare_access_checks(struct damon_ctx *ctx);
> > +unsigned int damon_va_check_accesses(struct damon_ctx *ctx);
> > +bool damon_va_target_valid(void *t);
> > +void damon_va_cleanup(struct damon_ctx *ctx);
> > +void damon_va_set_primitives(struct damon_ctx *ctx);
> 
> Any reason for these to be in the header?

To let DAMON API users (in kernel space) and other primitives developers to use
those.

> 
> > +
> [snip]
> > +
> > +static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
> > +                       unsigned long *page_sz)
> > +{
> > +       pte_t *pte = NULL;
> > +       pmd_t *pmd = NULL;
> > +       spinlock_t *ptl;
> > +       bool young = false;
> > +
> > +       if (follow_pte_pmd(mm, addr, NULL, &pte, &pmd, &ptl))
> > +               return false;
> > +
> > +       *page_sz = PAGE_SIZE;
> > +       if (pte) {
> > +               young = pte_young(*pte);
> > +               if (!young)
> > +                       young = !page_is_idle(pte_page(*pte));
> > +               pte_unmap_unlock(pte, ptl);
> > +               return young;
> > +       }
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +       young = pmd_young(*pmd);
> > +       if (!young)
> > +               young = !page_is_idle(pmd_page(*pmd));
> > +       spin_unlock(ptl);
> > +       *page_sz = ((1UL) << HPAGE_PMD_SHIFT);
> > +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> > +
> > +       return young;
> 
> You need mmu_notifier_test_young() here. Hmm I remember mentioning
> this in some previous version as well.

Your question and my answer was as below:

    > Don't you need mmu_notifier_clear_young() here?

    I think we don't need it here because we only read the Accessed bit and PG_Idle
    if Accessed bit was not set.

I should notice that you mean 'test_young()' but didn't, sorry.  I will add it
in the next version.

> 
> BTW have you tested this on a VM?

Yes.  Indeed, I'm testing this on a QEMU/KVM environment.  You can get more
detail at: https://damonitor.github.io/doc/html/latest/vm/damon/eval.html#setup

> 
> The patch looks good overall.

Thanks.  I will fix above things in the next version.


Thanks,
SeongJae Park