linux-kernel - Re: [PATCH v2] efi/unaccepted: Fix off-by-one when checking for overlapping ranges

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Date:   Fri, 3 Nov 2023 16:30:01 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michael Roth <michael.roth@....com>, linux-efi@...r.kernel.org
Cc:     x86@...nel.org, linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nikolay Borisov <nik.borisov@...e.com>, stable@...nel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2] efi/unaccepted: Fix off-by-one when checking for
 overlapping ranges

On 11/3/23 16:13, Michael Roth wrote:
> When a task needs to accept memory it will scan the accepting_list
> to see if any ranges already being processed by other tasks overlap
> with its range. Due to an off-by-one in the range comparisons, a task
> might falsely determine that an overlapping range is being accepted,
> leading to an unnecessary delay before it begins processing the range.
> 
> Fix the off-by-one in the range comparison to prevent this and slightly
> improve performance.
> 
> Fixes: 50e782a86c98 ("efi/unaccepted: Fix soft lockups caused by parallel memory acceptance")
> Link: https://lore.kernel.org/linux-mm/20231101004523.vseyi5bezgfaht5i@amd.com/T/#me2eceb9906fcae5fe958b3fe88e41f920f8335b6
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Signed-off-by: Michael Roth <michael.roth@....com>

More justification for introducing a common ranges_overlap() helper somewhere :)

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

> ---
> v2:
>  * Improve commit message terminology (Kirill)
> ---
>  drivers/firmware/efi/unaccepted_memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/efi/unaccepted_memory.c b/drivers/firmware/efi/unaccepted_memory.c
> index 135278ddaf62..79fb687bb90f 100644
> --- a/drivers/firmware/efi/unaccepted_memory.c
> +++ b/drivers/firmware/efi/unaccepted_memory.c
> @@ -100,7 +100,7 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
>  	 * overlap on physical address level.
>  	 */
>  	list_for_each_entry(entry, &accepting_list, list) {
> -		if (entry->end < range.start)
> +		if (entry->end <= range.start)
>  			continue;
>  		if (entry->start >= range.end)
>  			continue;