linux-kernel - Re: [PATCH v7 02/20] fs/dax: Return unmapped busy pages from dax_layout_busy_page

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Message-ID: <d998abb2-e825-4f2b-9bd6-6a0465abf123@nvidia.com>
Date: Wed, 5 Feb 2025 14:15:13 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
 dan.j.williams@...el.com, linux-mm@...ck.org
Cc: Alison Schofield <alison.schofield@...el.com>, lina@...hilina.net,
 zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com,
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 dave.hansen@...ux.intel.com, ira.weiny@...el.com, willy@...radead.org,
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 loongarch@...ts.linux.dev
Subject: Re: [PATCH v7 02/20] fs/dax: Return unmapped busy pages from
 dax_layout_busy_page_range()

On 2/5/25 09:47, Alistair Popple wrote:
> dax_layout_busy_page_range() is used by file systems to scan the DAX
> page-cache to unmap mapping pages from user-space and to determine if
> any pages in the given range are busy, either due to ongoing DMA or
> other get_user_pages() usage.
> 
> Currently it checks to see the file mapping is mapped into user-space
> with mapping_mapped() and returns early if not, skipping the check for
> DMA busy pages. This is wrong as pages may still be undergoing DMA
> access even if they have subsequently been unmapped from
> user-space. Fix this by dropping the check for mapping_mapped().
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  fs/dax.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 21b4740..5133568 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -690,7 +690,7 @@ struct page *dax_layout_busy_page_range(struct address_space *mapping,
>  	if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
>  		return NULL;
>  
> -	if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> +	if (!dax_mapping(mapping))
>  		return NULL;
>  
>  	/* If end == LLONG_MAX, all pages from start to till end of file */

I think the patch should probably also add

if (mapping_mapped(mapping))
	unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1, 0);
But I don't think it's a blocker unmap_mapping_pages() should do the right thing internally

Acked-by: Balbir Singh <balbirs@...dia.com>