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

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