linux-kernel - Re: [PATCH 08/10] mm/mremap: share the i_mmap

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Message-ID: <alpine.LSU.2.11.1411032148230.15596@eggly.anvils>
Date:	Mon, 3 Nov 2014 22:04:24 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	"Kirill A. Shutemov" <kirill.shutemov@...el.linux.com>,
	Michel Lespinasse <walken@...gle.com>,
	akpm@...ux-foundation.org, hughd@...gle.com, riel@...hat.com,
	mgorman@...e.de, peterz@...radead.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, dbueso@...e.de, linux-mm@...ck.org
Subject: Re: [PATCH 08/10] mm/mremap: share the i_mmap_rwsem

I'm glad to see this series back, and nicely presented: thank you.
Not worth respinning them, but consider 1,2,3,4,5,6,7 and 9 as
Acked-by: Hugh Dickins <hughd@...gle.com>

On Thu, 30 Oct 2014, Davidlohr Bueso wrote:

> As per the comment in move_ptes(), we only require taking the
> anon vma and i_mmap locks to ensure that rmap will always observe
> either the old or new ptes, in the case of need_rmap_lock=true.
> No modifications to the tree itself, thus share the i_mmap_rwsem.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...el.linux.com>

But this one is Nacked by me.  I don't understand how you and Kirill
could read Michel's painstaking comment on need_rmap_locks, then go
go ahead and remove the exclusion of rmap_walk().

I agree the code here does not modify the interval tree, but the
comment explains how we're moving a pte from one place in the tree
to another, and in some cases there's a danger that the rmap walk
might miss the pte from both places (which doesn't matter much to
most of its uses, but is critical in page migration).

Or am I the one missing something?

Hugh

> ---
>  mm/mremap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index c929324..09bd644 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -119,7 +119,7 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>  	if (need_rmap_locks) {
>  		if (vma->vm_file) {
>  			mapping = vma->vm_file->f_mapping;
> -			i_mmap_lock_write(mapping);
> +			i_mmap_lock_read(mapping);
>  		}
>  		if (vma->anon_vma) {
>  			anon_vma = vma->anon_vma;
> @@ -156,7 +156,7 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>  	if (anon_vma)
>  		anon_vma_unlock_read(anon_vma);
>  	if (mapping)
> -		i_mmap_unlock_write(mapping);
> +		i_mmap_unlock_read(mapping);
>  }
>  
>  #define LATENCY_LIMIT	(64 * PAGE_SIZE)
> -- 
> 1.8.4.5
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