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

[David Hildenbrand <david@redhat.com>]

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().

You can exclude any VMAs you don't care about in the test_walk() 
callback, if required.

-- 
Thanks,

David / dhildenb