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From:   Tianxianting <tian.xianting@h3c.com>
To:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>
CC:     "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] mm: bio_alloc never fails when set GFP_NOIO, GFP_KERNEL
Thread-Topic: [PATCH] mm: bio_alloc never fails when set GFP_NOIO, GFP_KERNEL
Thread-Index: AQHWp1lEOcaxBjwhyUKPdENxUdfPnqmlZyxg
Date:   Fri, 23 Oct 2020 16:40:01 +0000
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References: <20201021031128.14100-1-tian.xianting@h3c.com>
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Hi Andrew
Could I get your comments for this patch?  Thanks in advance.

-----Original Message-----
From: tianxianting (RD) 
Sent: Wednesday, October 21, 2020 11:11 AM
To: akpm@linux-foundation.org
Cc: linux-mm@kvack.org; linux-kernel@vger.kernel.org; tianxianting (RD) <tian.xianting@h3c.com>
Subject: [PATCH] mm: bio_alloc never fails when set GFP_NOIO, GFP_KERNEL

bio_alloc with __GFP_DIRECT_RECLAIM(which is included in GFP_NOIO,
GFP_KERNEL) never fails, as stated in the comments of bio_alloc_bioset.

So we can remove multiple unneeded null checks of bio_alloc and simplify the code.

We have done it in fs/ext4/readpage.c, fs/ext4/page-io.c, fs/direct-io.c, and so forth.

Signed-off-by: Xianting Tian <tian.xianting@h3c.com>
---
 mm/page_io.c | 31 +++++++++++--------------------
 1 file changed, 11 insertions(+), 20 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c index e485a6e8a..9215bb356 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -30,18 +30,20 @@ static struct bio *get_swap_bio(gfp_t gfp_flags,
 				struct page *page, bio_end_io_t end_io)  {
 	struct bio *bio;
+	struct block_device *bdev;
 
+	/*
+	 * bio_alloc will _always_ be able to allocate a bio if
+	 * __GFP_DIRECT_RECLAIM is set, see comments for bio_alloc_bioset().
+	 */
 	bio = bio_alloc(gfp_flags, 1);
-	if (bio) {
-		struct block_device *bdev;
+	bio->bi_iter.bi_sector = map_swap_page(page, &bdev);
+	bio_set_dev(bio, bdev);
+	bio->bi_iter.bi_sector <<= PAGE_SHIFT - 9;
+	bio->bi_end_io = end_io;
 
-		bio->bi_iter.bi_sector = map_swap_page(page, &bdev);
-		bio_set_dev(bio, bdev);
-		bio->bi_iter.bi_sector <<= PAGE_SHIFT - 9;
-		bio->bi_end_io = end_io;
+	bio_add_page(bio, page, thp_size(page), 0);
 
-		bio_add_page(bio, page, thp_size(page), 0);
-	}
 	return bio;
 }
 
@@ -351,19 +353,13 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 
 	ret = 0;
 	bio = get_swap_bio(GFP_NOIO, page, end_write_func);
-	if (bio == NULL) {
-		set_page_dirty(page);
-		unlock_page(page);
-		ret = -ENOMEM;
-		goto out;
-	}
 	bio->bi_opf = REQ_OP_WRITE | REQ_SWAP | wbc_to_write_flags(wbc);
 	bio_associate_blkg_from_page(bio, page);
 	count_swpout_vm_event(page);
 	set_page_writeback(page);
 	unlock_page(page);
 	submit_bio(bio);
-out:
+
 	return ret;
 }
 
@@ -416,11 +412,6 @@ int swap_readpage(struct page *page, bool synchronous)
 
 	ret = 0;
 	bio = get_swap_bio(GFP_KERNEL, page, end_swap_bio_read);
-	if (bio == NULL) {
-		unlock_page(page);
-		ret = -ENOMEM;
-		goto out;
-	}
 	disk = bio->bi_disk;
 	/*
 	 * Keep this task valid during swap readpage because the oom killer may
--
2.17.1