Re: [PATCH] mm/damon/vaddr: Safely walk page table

[SeongJae Park]

From: SeongJae Park <sjpark@xxxxxxxxx>

On Tue, 31 Aug 2021 11:53:05 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:

> On 27.08.21 17:04, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@xxxxxxxxx>
> > 
> > Commit d7f647622761 ("mm/damon: implement primitives for the virtual
> > memory address spaces") of linux-mm[1] tries to find PTE or PMD for
> > arbitrary virtual address using 'follow_invalidate_pte()' without proper
> > locking[2].  This commit fixes the issue by using another page table
> > walk function for more general use case under proper locking.
> > 
> > [1] https://github.com/hnaz/linux-mm/commit/d7f647622761
> > [2] https://lore.kernel.org/linux-mm/3b094493-9c1e-6024-bfd5-7eca66399b7e@xxxxxxxxxx
> > 
> > Fixes: d7f647622761 ("mm/damon: implement primitives for the virtual memory address spaces")
> > Reported-by: David Hildenbrand <david@xxxxxxxxxx>
> > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> > ---
> >   mm/damon/vaddr.c | 81 +++++++++++++++++++++++++++++++++++++++++++-----
> >   1 file changed, 74 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> > index 230db7413278..b3677f2ef54b 100644
> > --- a/mm/damon/vaddr.c
> > +++ b/mm/damon/vaddr.c
> > @@ -8,10 +8,12 @@
> >   #define pr_fmt(fmt) "damon-va: " fmt
> >   
> >   #include <linux/damon.h>
> > +#include <linux/hugetlb.h>
> >   #include <linux/mm.h>
> >   #include <linux/mmu_notifier.h>
> >   #include <linux/highmem.h>
> >   #include <linux/page_idle.h>
> > +#include <linux/pagewalk.h>
> >   #include <linux/random.h>
> >   #include <linux/sched/mm.h>
> >   #include <linux/slab.h>
> > @@ -446,14 +448,69 @@ static void damon_pmdp_mkold(pmd_t *pmd, struct mm_struct *mm,
> >   #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> >   }
> >   
> > +struct damon_walk_private {
> > +	pmd_t *pmd;
> > +	pte_t *pte;
> > +	spinlock_t *ptl;
> > +};
> > +
> > +static int damon_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
> > +		struct mm_walk *walk)
> > +{
> > +	struct damon_walk_private *priv = walk->private;
> > +
> > +	if (pmd_huge(*pmd)) {
> > +		priv->ptl = pmd_lock(walk->mm, pmd);
> > +		if (pmd_huge(*pmd)) {
> > +			priv->pmd = pmd;
> > +			return 0;
> > +		}
> > +		spin_unlock(priv->ptl);
> > +	}
> > +
> > +	if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
> > +		return -EINVAL;
> > +	priv->pte = pte_offset_map_lock(walk->mm, pmd, addr, &priv->ptl);
> > +	if (!pte_present(*priv->pte)) {
> > +		pte_unmap_unlock(priv->pte, priv->ptl);
> > +		priv->pte = NULL;
> > +		return -EINVAL;
> > +	}
> > +	return 0;
> > +}
> > +
> > +static struct mm_walk_ops damon_walk_ops = {
> > +	.pmd_entry = damon_pmd_entry,
> > +};
> > +
> > +int damon_follow_pte_pmd(struct mm_struct *mm, unsigned long addr,
> > +		struct damon_walk_private *private)
> > +{
> > +	int rc;
> > +
> > +	private->pte = NULL;
> > +	private->pmd = NULL;
> > +	rc = walk_page_range(mm, addr, addr + 1, &damon_walk_ops, private);
> > +	if (!rc && !private->pte && !private->pmd)
> > +		return -EINVAL;
> > +	return rc;
> > +}
> > +
> >   static void damon_va_mkold(struct mm_struct *mm, unsigned long addr)
> >   {
> > -	pte_t *pte = NULL;
> > -	pmd_t *pmd = NULL;
> > +	struct damon_walk_private walk_result;
> > +	pte_t *pte;
> > +	pmd_t *pmd;
> >   	spinlock_t *ptl;
> >   
> > -	if (follow_invalidate_pte(mm, addr, NULL, &pte, &pmd, &ptl))
> > +	mmap_write_lock(mm);
> 
> Can you elaborate why mmap_read_lock() isn't sufficient for your use 
> case? The write mode might heavily affect damon performance and workload 
> impact.

Because as you also mentioned in the previous mail, 'we can walk page tables
ignoring VMAs with the mmap semaphore held in write mode', and in this case we
don't know to which VMA the address is belong.  I thought the link to the mail
can help people understanding the reason.  But, as you are suggesting, I now
think putting an elaborated explanation here would be much better.  I will also
put a warning for the possible performance impact.

> 
> 
> Also, I wonder if it wouldn't be much easier and cleaner to just handle 
> it completely in the .pmd_entry callback, instead of returning PMDs, 
> PTEs, LOCKs, ... here.
> 
> You could have
> 
> static struct mm_walk_ops damon_mkold_ops = {
> 	.pmd_entry = damon_mkold_pmd_entry,
> };
> 
> and
> 
> static struct mm_walk_ops damon_young_ops = {
> 	.pmd_entry = damon_young_pmd_entry,
> };
> 
> And then just handle everything completely inside the callback, avoiding 
> having to return locked PTEs, PMDs, .... and instead handling it at a 
> single location. Simply forward the page_sz pointer in the latter case 
> to damon_young_ops.
> 
> 
> damon_va_mkold()/damon_va_young() would mostly only call 
> walk_page_range() with the right ops and eventually convert some return 
> values.

I just wanted to make the change as small as possible, but you're right.  That
must be much cleaner.  I will post the next version soon.


Thanks,
SeongJae Park

> 
> -- 
> Thanks,
> 
> David / dhildenb