linux-ext4 - Re: [PATCH v2 1/3] migrate.c: Remove vma check in migrate_vma

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Date:   Tue, 15 Feb 2022 13:50:44 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org
Cc:     Felix.Kuehling@....com, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jgg@...dia.com, jglisse@...hat.com,
        willy@...radead.org, alex.sierra@....com
Subject: Re: [PATCH v2 1/3] migrate.c: Remove vma check in migrate_vma_setup()

On 2/6/22 20:26, Alistair Popple wrote:
> migrate_vma_setup() checks that a valid vma is passed so that the page
> tables can be walked to find the pfns associated with a given address
> range. However in some cases the pfns are already known, such as when
> migrating device coherent pages during pin_user_pages() meaning a valid
> vma isn't required.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Acked-by: Felix Kuehling <Felix.Kuehling@....com>
> ---
> 
> Changes for v2:
> 
>   - Added Felix's Acked-by
> 
>   mm/migrate.c | 34 +++++++++++++++++-----------------
>   1 file changed, 17 insertions(+), 17 deletions(-)
> 

Hi Alistair,

Another late-breaking review question, below. :)

> diff --git a/mm/migrate.c b/mm/migrate.c
> index a9aed12..0d6570d 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2602,24 +2602,24 @@ int migrate_vma_setup(struct migrate_vma *args)
>   
>   	args->start &= PAGE_MASK;
>   	args->end &= PAGE_MASK;
> -	if (!args->vma || is_vm_hugetlb_page(args->vma) ||
> -	    (args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
> -		return -EINVAL;
> -	if (nr_pages <= 0)
> -		return -EINVAL;

Was the above check left out intentionally? If so, then it needs a
commit description note. And maybe even a separate patch, because it
changes the behavior.

If you do want to change the behavior:

* The kerneldoc comment above this function supports such a change: it
requires returning 0 for the case of zero pages requested. So your
change would bring the comments into alignment with the code.

* I don't think memset deals properly with a zero length input arg, so
it's probably better to return 0, before that point.


thanks,
-- 
John Hubbard
NVIDIA

> -	if (args->start < args->vma->vm_start ||
> -	    args->start >= args->vma->vm_end)
> -		return -EINVAL;
> -	if (args->end <= args->vma->vm_start || args->end > args->vma->vm_end)
> -		return -EINVAL;
>   	if (!args->src || !args->dst)
>   		return -EINVAL;
> -
> -	memset(args->src, 0, sizeof(*args->src) * nr_pages);
> -	args->cpages = 0;
> -	args->npages = 0;
> -
> -	migrate_vma_collect(args);
> +	if (args->vma) {
> +		if (is_vm_hugetlb_page(args->vma) ||
> +			(args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
> +			return -EINVAL;
> +		if (args->start < args->vma->vm_start ||
> +			args->start >= args->vma->vm_end)
> +			return -EINVAL;
> +		if (args->end <= args->vma->vm_start || args->end > args->vma->vm_end)
> +			return -EINVAL;
> +
> +		memset(args->src, 0, sizeof(*args->src) * nr_pages);
> +		args->cpages = 0;
> +		args->npages = 0;
> +
> +		migrate_vma_collect(args);
> +	}
>   
>   	if (args->cpages)
>   		migrate_vma_unmap(args);
> @@ -2804,7 +2804,7 @@ void migrate_vma_pages(struct migrate_vma *migrate)
>   			continue;
>   		}
>   
> -		if (!page) {
> +		if (!page && migrate->vma) {
>   			if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
>   				continue;
>   			if (!notified) {