linux-kernel - Re: [PATCH v2 6/9] hugetlb: add vma based lock for pmd sharing

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Message-ID: <2b1b6d09-0188-23a3-6ac3-6e81446a10e4@huawei.com>
Date:   Sat, 24 Sep 2022 21:11:48 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
CC:     Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Sven Schnelle <svens@...ux.ibm.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>
Subject: Re: [PATCH v2 6/9] hugetlb: add vma based lock for pmd sharing

On 2022/9/15 6:18, Mike Kravetz wrote:
> Allocate a new hugetlb_vma_lock structure and hang off vm_private_data
> for synchronization use by vmas that could be involved in pmd sharing.
> This data structure contains a rw semaphore that is the primary tool
> used for synchronization.
> 
> This new structure is ref counted, so that it can exist when NOT attached
> to a vma.  This is only helpful in resolving lock ordering issues where
> code may need to obtain the vma_lock while there are no guarantees the
> vma may go away.  By obtaining a ref on the structure, it can be
> guaranteed that at least the rw semaphore will not go away.
> 
> Only add infrastructure for the new lock here.  Actual use will be added
> in subsequent patches.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

LGTM with some nits below. Thanks for your work, Mike.

Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>

> -/* Reset counters to 0 and clear all HPAGE_RESV_* flags */
> -void reset_vma_resv_huge_pages(struct vm_area_struct *vma)
> +void hugetlb_dup_vma_private(struct vm_area_struct *vma)
>  {
>  	VM_BUG_ON_VMA(!is_vm_hugetlb_page(vma), vma);
> +	/*
> +	 * Clear vm_private_data
> +	 * - For MAP_PRIVATE mappings, this is the reserve map which does
> +	 *   not apply to children.  Faults generated by the children are
> +	 *   not guaranteed to succeed, even if read-only.
> +	 * - For shared mappings this is a per-vma semaphore that may be
> +	 *   allocated in a subsequent call to hugetlb_vm_op_open.
> +	 */
> +	vma->vm_private_data = (void *)0;
>  	if (!(vma->vm_flags & VM_MAYSHARE))
> -		vma->vm_private_data = (void *)0;
> +		return;

This if block can be deleted ? It doesn't do anything here.

>  }
>  
>  /*

<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 how VM_SHARED could
> +	 * be set without VM_MAYSHARE.  As a result, we need to
> +	 * check if either is set in the free path.
> +	 */
> +	if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
> +		return;
> +
> +	if (vma->vm_private_data) {
> +		struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
> +
> +		/*
> +		 * vma_lock structure may or not be released, but it

may or not be released?

Thanks,
Miaohe Lin