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Date:   Tue, 18 Aug 2020 10:15:24 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] block: Fix page_is_mergeable() for compound pages
Message-ID: <20200818021524.GA2505641@T590>
References: <20200817195206.15172-1-willy@infradead.org>
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On Mon, Aug 17, 2020 at 08:52:06PM +0100, Matthew Wilcox (Oracle) wrote:
> If we pass in an offset which is larger than PAGE_SIZE, then
> page_is_mergeable() thinks it's not mergeable with the previous bio_vec,
> leading to a large number of bio_vecs being used.  Use a slightly more
> obvious test that the two pages are compatible with each other.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Fixes: 52d52d1c98a9 ("block: only allow contiguous page structs in a bio_vec")
> 
> ---
>  block/bio.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index c63ba04bd629..a9931f23d933 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -740,8 +740,8 @@ static inline bool page_is_mergeable(const struct bio_vec *bv,
>  		struct page *page, unsigned int len, unsigned int off,
>  		bool *same_page)
>  {
> -	phys_addr_t vec_end_addr = page_to_phys(bv->bv_page) +
> -		bv->bv_offset + bv->bv_len - 1;
> +	size_t bv_end = bv->bv_offset + bv->bv_len;
> +	phys_addr_t vec_end_addr = page_to_phys(bv->bv_page) + bv_end - 1;
>  	phys_addr_t page_addr = page_to_phys(page);
>  
>  	if (vec_end_addr + 1 != page_addr + off)
> @@ -750,9 +750,9 @@ static inline bool page_is_mergeable(const struct bio_vec *bv,
>  		return false;
>  
>  	*same_page = ((vec_end_addr & PAGE_MASK) == page_addr);
> -	if (!*same_page && pfn_to_page(PFN_DOWN(vec_end_addr)) + 1 != page)
> -		return false;
> -	return true;
> +	if (*same_page)
> +		return true;
> +	return (bv->bv_page + bv_end / PAGE_SIZE) == (page + off / PAGE_SIZE);

Looks this way is more straightforward, meantime can cover compound
pages:

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming