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

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Date:   Wed, 8 Nov 2023 12:21:25 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Michael Roth <michael.roth@....com>, linux-efi@...r.kernel.org,
        x86@...nel.org, linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.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 Fri, 3 Nov 2023 at 16:30, Vlastimil Babka <vbabka@...e.cz> wrote:
>
> 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>
>

Thanks, I'll take this as a fix.


> > ---
> > 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;
>