Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH] mm/damon/vaddr: Safely walk page table | Date | Tue, 31 Aug 2021 11:53:05 +0200 |
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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.
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.
-- Thanks,
David / dhildenb
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