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From:   SeongJae Park <sj38.park@gmail.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     SeongJae Park <sj38.park@gmail.com>, akpm@linux-foundation.org,
        markubo@amazon.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, SeongJae Park <sjpark@amazon.de>
Subject: Re: [PATCH] mm/damon/vaddr: Safely walk page table
Date:   Tue, 31 Aug 2021 11:56:03 +0000
Message-Id: <20210831115603.28974-1-sjpark@amazon.de>
In-Reply-To: <d92a357b-c64c-aaf8-a69a-f00f93022013@redhat.com>
Precedence: bulk

From: SeongJae Park <sjpark@amazon.de>

On Tue, 31 Aug 2021 13:46:42 +0200 David Hildenbrand <david@redhat.com> wrote:

> On 31.08.21 12:49, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> > 
> > On Tue, 31 Aug 2021 11:53:05 +0200 David Hildenbrand <david@redhat.com> wrote:
> > 
> >> On 27.08.21 17:04, SeongJae Park wrote:
> >>> From: SeongJae Park <sjpark@amazon.de>
> >>>
> >>> 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@redhat.com
> >>>
> >>> Fixes: d7f647622761 ("mm/damon: implement primitives for the virtual memory address spaces")
> >>> Reported-by: David Hildenbrand <david@redhat.com>
> >>> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> >>> ---
> >>>    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.
> 
> walk_page_range() make sure to skip any VMA holes and only walks ranges 
> within VMAs. With the mmap sem in read mode, the VMA layout (mostly) 
> cannot change, so calling walk_page_range() is fine. So pagewalk.c 
> properly takes care of VMAs.
> 
> As an example, take a look at MADV_COLD handling in mm/madvise.c.
> 
> madvise_need_mmap_write() returns "0", and we end up calling 
> madvise_cold()->...->walk_page_range() with mmap_lock_read().

Oops, can't believe how I missed that.  I will hold only mmap read lock...


Thanks,
SJ

> 
> You can exclude any VMAs you don't care about in the test_walk() 
> callback, if required.
> 
> -- 
> Thanks,
> 
> David / dhildenb