linux-kernel - Re: [PATCH 04/10] mm/mremap: cleanup post-processing stage of mremap

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Message-ID: <277aa9f4-ddb7-4f34-ad15-a98888108cb5@suse.cz>
Date: Thu, 10 Jul 2025 15:49:09 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Peter Xu <peterx@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
 linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 04/10] mm/mremap: cleanup post-processing stage of mremap

On 7/7/25 07:27, Lorenzo Stoakes wrote:
> Separate out the uffd bits so it clear's what's happening.
> 
> Don't bother setting vrm->mmap_locked after unlocking, because after this
> we are done anyway.
> 
> The only time we drop the mmap lock is on VMA shrink, at which point
> vrm->new_len will be < vrm->old_len and the operation will not be performed
> anyway, so move this code out of the if (vrm->mmap_locked) block.
> 
> All addresses returned by mremap() are page-aligned, so the
> offset_in_page() check on ret seems only to be incorrectly trying to detect

"incorrectly" to me implies there's a bug. But AFAIU there's not, so maybe
e.g. "inappropriately"?

> whether an error occurred - explicitly check for this.
> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Just a nit:

> ---
>  mm/mremap.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 60eb0ac8634b..660bdb75e2f9 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -1729,6 +1729,15 @@ static int check_prep_vma(struct vma_remap_struct *vrm)
>  	return 0;
>  }
>  
> +static void notify_uffd(struct vma_remap_struct *vrm, unsigned long ret)

"ret" not "res"? :) Or actually why not name it for what it is,
mremap_userfaultfd_complete() names the parameter "to". Maybe to_addr or
new_addr?

> +{
> +	struct mm_struct *mm = current->mm;
> +
> +	userfaultfd_unmap_complete(mm, vrm->uf_unmap_early);
> +	mremap_userfaultfd_complete(vrm->uf, vrm->addr, ret, vrm->old_len);
> +	userfaultfd_unmap_complete(mm, vrm->uf_unmap);
> +}
> +
>  static unsigned long do_mremap(struct vma_remap_struct *vrm)
>  {
>  	struct mm_struct *mm = current->mm;
> @@ -1754,18 +1763,13 @@ static unsigned long do_mremap(struct vma_remap_struct *vrm)
>  	res = vrm_implies_new_addr(vrm) ? mremap_to(vrm) : mremap_at(vrm);
>  
>  out:
> -	if (vrm->mmap_locked) {
> +	if (vrm->mmap_locked)
>  		mmap_write_unlock(mm);
> -		vrm->mmap_locked = false;
> -
> -		if (!offset_in_page(res) && vrm->mlocked && vrm->new_len > vrm->old_len)
> -			mm_populate(vrm->new_addr + vrm->old_len, vrm->delta);
> -	}
>  
> -	userfaultfd_unmap_complete(mm, vrm->uf_unmap_early);
> -	mremap_userfaultfd_complete(vrm->uf, vrm->addr, res, vrm->old_len);
> -	userfaultfd_unmap_complete(mm, vrm->uf_unmap);
> +	if (!IS_ERR_VALUE(res) && vrm->mlocked && vrm->new_len > vrm->old_len)
> +		mm_populate(vrm->new_addr + vrm->old_len, vrm->delta);
>  
> +	notify_uffd(vrm, res);
>  	return res;
>  }
>