linux-kernel - Re: [PATCH 6/8] hugetlb: add vma based lock for pmd sharing

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Message-ID: <47cc90bf-d616-5004-555d-b3d7e9b09bd1@huawei.com>
Date:   Sat, 27 Aug 2022 17:30:27 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Ray Fucillo <Ray.Fucillo@...ersystems.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/8] hugetlb: add vma based lock for pmd sharing

On 2022/8/25 1:57, Mike Kravetz wrote:
> Allocate a rw semaphore and hang off vm_private_data for
> synchronization use by vmas that could be involved in pmd sharing.  Only
> add infrastructure for the new lock here.  Actual use will be added in
> subsequent patch.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

<snip>

> +static void hugetlb_vma_lock_free(struct vm_area_struct *vma)
> +{
> +	/*
> +	 * Only present in sharable vmas.  See comment in
> +	 * __unmap_hugepage_range_final about the neeed to check both

s/neeed/need/

> +	 * VM_SHARED and VM_MAYSHARE in free path

I think there might be some wrong checks around this patch. As above comment said, we
need to check both flags, so we should do something like below instead?

	if (!(vma->vm_flags & (VM_MAYSHARE | VM_SHARED) == (VM_MAYSHARE | VM_SHARED)))

> +	 */
> +	if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
> +		return;
> +
> +	if (vma->vm_private_data) {
> +		kfree(vma->vm_private_data);
> +		vma->vm_private_data = NULL;
> +	}
> +}
> +
> +static void hugetlb_vma_lock_alloc(struct vm_area_struct *vma)
> +{
> +	struct rw_semaphore *vma_sema;
> +
> +	/* Only establish in (flags) sharable vmas */
> +	if (!vma || !(vma->vm_flags & VM_MAYSHARE))
> +		return;
> +
> +	/* Should never get here with non-NULL vm_private_data */

We can get here with non-NULL vm_private_data when called from hugetlb_vm_op_open during fork?

Also there's one missing change on comment:

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d0617d64d718..4bc844a1d312 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -863,7 +863,7 @@ __weak unsigned long vma_mmu_pagesize(struct vm_area_struct *vma)
  * faults in a MAP_PRIVATE mapping. Only the process that called mmap()
  * is guaranteed to have their future faults succeed.
  *
- * With the exception of reset_vma_resv_huge_pages() which is called at fork(),
+ * With the exception of hugetlb_dup_vma_private() which is called at fork(),
  * the reserve counters are updated with the hugetlb_lock held. It is safe
  * to reset the VMA at fork() time as it is not in use yet and there is no
  * chance of the global counters getting corrupted as a result of the values.


Otherwise this patch looks good to me. Thanks.

Thanks,
Miaohe Lin