linux-kernel - Re: [PATCH v2 2/3] mm: rmap: Move the cache flushing to the correct place for hugetlb PMD sharing

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Message-ID: <YmosZpGwSoB8TlnK@FVFYT0MHHV2J.usts.net>
Date:   Thu, 28 Apr 2022 13:55:50 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        almasrymina@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] mm: rmap: Move the cache flushing to the correct
 place for hugetlb PMD sharing

On Wed, Apr 27, 2022 at 06:52:06PM +0800, Baolin Wang wrote:
> The cache level flush will always be first when changing an existing
> virtual–>physical mapping to a new value, since this allows us to
> properly handle systems whose caches are strict and require a
> virtual–>physical translation to exist for a virtual address. So we
> should move the cache flushing before huge_pmd_unshare().
>

Right.

> As Muchun pointed out[1], now the architectures whose supporting hugetlb
> PMD sharing have no cache flush issues in practice. But I think we
> should still follow the cache/TLB flushing rules when changing a valid
> virtual address mapping in case of potential issues in future.

Right. One point i need to clarify. I do not object this change but
want you to clarify this (not an issue in practice) in commit log
to let others know they do not need to bp this.

> 
> [1] https://lore.kernel.org/all/YmT%2F%2FhuUbFX+KHcy@FVFYT0MHHV2J.usts.net/
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  mm/rmap.c | 40 ++++++++++++++++++++++------------------
>  1 file changed, 22 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 61e63db..4f0d115 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1535,15 +1535,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  			 * do this outside rmap routines.
>  			 */
>  			VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
> +			/*
> +			 * huge_pmd_unshare may unmap an entire PMD page.
> +			 * There is no way of knowing exactly which PMDs may
> +			 * be cached for this mm, so we must flush them all.
> +			 * start/end were already adjusted above to cover this
> +			 * range.
> +			 */
> +			flush_cache_range(vma, range.start, range.end);
> +

flush_cache_range() is always called even if we do not need to flush.
How about introducing a new helper like hugetlb_pmd_shared() which
returns true for shared PMD? Then:

	if (hugetlb_pmd_shared(mm, vma, pvmw.pte)) {
		flush_cache_range(vma, range.start, range.end);
		huge_pmd_unshare(mm, vma, &address, pvmw.pte);
		flush_tlb_range(vma, range.start, range.end);
	}

The code could be a little simpler. Right?

Thanks.