linux-kernel - Re: [PATCH] iomap: Fix the write_count in iomap_add_to

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Date:   Fri, 21 Aug 2020 07:07:10 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Anju T Sudhakar <anju@...ux.vnet.ibm.com>
Cc:     hch@...radead.org, darrick.wong@...cle.com,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, willy@...radead.org,
        riteshh@...ux.ibm.com, linux-block@...r.kernel.org
Subject: Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend().

On Wed, Aug 19, 2020 at 03:58:41PM +0530, Anju T Sudhakar wrote:
> From: Ritesh Harjani <riteshh@...ux.ibm.com>
> 
> __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;