Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend().

[Jens Axboe]

On 8/21/20 12:07 AM, Christoph Hellwig wrote:
> On Wed, Aug 19, 2020 at 03:58:41PM +0530, Anju T Sudhakar wrote:
>> From: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
>>
>> __bio_try_merge_page() may return same_page = 1 and merged = 0. 
>> This could happen when bio->bi_iter.bi_size + len > UINT_MAX. 
>> Handle this case in iomap_add_to_ioend() by incrementing write_count.
>> This scenario mostly happens where we have too much dirty data accumulated. 
>>
>> w/o the patch we hit below kernel warning,
> 
> I think this is better fixed in the block layer rather than working
> around the problem in the callers.  Something like this:
> 
> diff --git a/block/bio.c b/block/bio.c
> index c63ba04bd62967..ef321cd1072e4e 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -879,8 +879,10 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>  		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
>  
>  		if (page_is_mergeable(bv, page, len, off, same_page)) {
> -			if (bio->bi_iter.bi_size > UINT_MAX - len)
> +			if (bio->bi_iter.bi_size > UINT_MAX - len) {
> +				*same_page = false;
>  				return false;
> +			}
>  			bv->bv_len += len;
>  			bio->bi_iter.bi_size += len;
>  			return true;
> 

This looks good to me.

-- 
Jens Axboe