[RFC PATCH v2 5/5] mm: Remove swap BIO paths and only use DIO paths [BROKEN]

[David Howells <dhowells@redhat.com>]

[!] NOTE: This doesn't work and might damage your disk's contents.

Delete the BIO-generating swap read/write paths and always use
->direct_IO().  This puts the mapping layer in the filesystem.

This doesn't work - probably due to ki_pos being set to
page_file_offset(page) which then gets remapped.

Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/direct-io.c      |    2 +
 include/linux/bio.h |    2 +
 mm/page_io.c        |  156 ++-------------------------------------------------
 3 files changed, 9 insertions(+), 151 deletions(-)

diff --git a/fs/direct-io.c b/fs/direct-io.c
index b2e86e739d7a..76eec0a68fa4 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -1216,6 +1216,8 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
 	}
 	if (iocb->ki_flags & IOCB_HIPRI)
 		dio->op_flags |= REQ_HIPRI;
+	if (iocb->ki_flags & IOCB_SWAP)
+		dio->op_flags |= REQ_SWAP;
 
 	/*
 	 * For AIO O_(D)SYNC writes we need to defer completions to a workqueue
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 2203b686e1f0..da75cfa72ed3 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -816,6 +816,8 @@ static inline void bio_set_polled(struct bio *bio, struct kiocb *kiocb)
 	bio->bi_opf |= REQ_HIPRI;
 	if (!is_sync_kiocb(kiocb))
 		bio->bi_opf |= REQ_NOWAIT;
+	if (kiocb->ki_flags & IOCB_SWAP)
+		bio->bi_opf |= REQ_SWAP;
 }
 
 struct bio *blk_next_bio(struct bio *bio, unsigned int nr_pages, gfp_t gfp);
diff --git a/mm/page_io.c b/mm/page_io.c
index dae7bbd7a842..fb260d9c3973 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -44,30 +44,6 @@ static void swapfile_put_kiocb(struct swapfile_kiocb *ki)
 	}
 }
 
-static void end_swap_bio_write(struct bio *bio)
-{
-	struct page *page = bio_first_page_all(bio);
-
-	if (bio->bi_status) {
-		SetPageError(page);
-		/*
-		 * We failed to write the page out to swap-space.
-		 * Re-dirty the page in order to avoid it being reclaimed.
-		 * Also print a dire warning that things will go BAD (tm)
-		 * very quickly.
-		 *
-		 * Also clear PG_reclaim to avoid folio_rotate_reclaimable()
-		 */
-		set_page_dirty(page);
-		pr_alert_ratelimited("Write-error on swap-device (%u:%u:%llu)\n",
-				     MAJOR(bio_dev(bio)), MINOR(bio_dev(bio)),
-				     (unsigned long long)bio->bi_iter.bi_sector);
-		ClearPageReclaim(page);
-	}
-	end_page_writeback(page);
-	bio_put(bio);
-}
-
 static void swap_slot_free_notify(struct page *page)
 {
 	struct swap_info_struct *sis;
@@ -116,32 +92,6 @@ static void swap_slot_free_notify(struct page *page)
 	}
 }
 
-static void end_swap_bio_read(struct bio *bio)
-{
-	struct page *page = bio_first_page_all(bio);
-	struct task_struct *waiter = bio->bi_private;
-
-	if (bio->bi_status) {
-		SetPageError(page);
-		ClearPageUptodate(page);
-		pr_alert_ratelimited("Read-error on swap-device (%u:%u:%llu)\n",
-				     MAJOR(bio_dev(bio)), MINOR(bio_dev(bio)),
-				     (unsigned long long)bio->bi_iter.bi_sector);
-		goto out;
-	}
-
-	SetPageUptodate(page);
-	swap_slot_free_notify(page);
-out:
-	unlock_page(page);
-	WRITE_ONCE(bio->bi_private, NULL);
-	bio_put(bio);
-	if (waiter) {
-		blk_wake_io_task(waiter);
-		put_task_struct(waiter);
-	}
-}
-
 int generic_swapfile_activate(struct swap_info_struct *sis,
 				struct file *swap_file,
 				sector_t *span)
@@ -281,31 +231,12 @@ static inline void count_swpout_vm_event(struct page *page)
 	count_vm_events(PSWPOUT, thp_nr_pages(page));
 }
 
-#if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
-static void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
-{
-	struct cgroup_subsys_state *css;
-	struct mem_cgroup *memcg;
-
-	memcg = page_memcg(page);
-	if (!memcg)
-		return;
-
-	rcu_read_lock();
-	css = cgroup_e_css(memcg->css.cgroup, &io_cgrp_subsys);
-	bio_associate_blkg_from_css(bio, css);
-	rcu_read_unlock();
-}
-#else
-#define bio_associate_blkg_from_page(bio, page)		do { } while (0)
-#endif /* CONFIG_MEMCG && CONFIG_BLK_CGROUP */
-
 static void __swapfile_write_complete(struct kiocb *iocb, long ret, long ret2)
 {
 	struct page *page = iocb->ki_swap_page;
 
 	if (ret == thp_size(page)) {
-		count_vm_event(PSWPOUT);
+		count_swpout_vm_event(page);
 		ret = 0;
 	} else {
 		/*
@@ -401,34 +332,10 @@ static int swapfile_write(struct swap_info_struct *sis,
 
 int __swap_writepage(struct page *page, struct writeback_control *wbc)
 {
-	struct bio *bio;
-	int ret;
 	struct swap_info_struct *sis = page_swap_info(page);
 
 	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
-	if (data_race(sis->flags & SWP_FS_OPS))
-		return swapfile_write(sis, page, wbc);
-
-	ret = bdev_write_page(sis->bdev, swap_page_sector(page), page, wbc);
-	if (!ret) {
-		count_swpout_vm_event(page);
-		return 0;
-	}
-
-	bio = bio_alloc(GFP_NOIO, 1);
-	bio_set_dev(bio, sis->bdev);
-	bio->bi_iter.bi_sector = swap_page_sector(page);
-	bio->bi_opf = REQ_OP_WRITE | REQ_SWAP | wbc_to_write_flags(wbc);
-	bio->bi_end_io = end_swap_bio_write;
-	bio_add_page(bio, page, thp_size(page), 0);
-
-	bio_associate_blkg_from_page(bio, page);
-	count_swpout_vm_event(page);
-	set_page_writeback(page);
-	unlock_page(page);
-	submit_bio(bio);
-
-	return 0;
+	return swapfile_write(sis, page, wbc);
 }
 
 static void __swapfile_read_complete(struct kiocb *iocb, long ret, long ret2)
@@ -437,6 +344,7 @@ static void __swapfile_read_complete(struct kiocb *iocb, long ret, long ret2)
 
 	if (ret == PAGE_SIZE) {
 		count_vm_event(PSWPIN);
+		swap_slot_free_notify(page);
 		SetPageUptodate(page);
 	} else {
 		SetPageError(page);
@@ -522,12 +430,9 @@ static int swapfile_read(struct swap_info_struct *sis, struct page *page,
 
 int swap_readpage(struct page *page, bool synchronous)
 {
-	struct bio *bio;
-	int ret = 0;
 	struct swap_info_struct *sis = page_swap_info(page);
-	blk_qc_t qc;
-	struct gendisk *disk;
 	unsigned long pflags;
+	int ret = 0;
 
 	VM_BUG_ON_PAGE(!PageSwapCache(page) && !synchronous, page);
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
@@ -543,60 +448,9 @@ int swap_readpage(struct page *page, bool synchronous)
 	if (frontswap_load(page) == 0) {
 		SetPageUptodate(page);
 		unlock_page(page);
-		goto out;
-	}
-
-	if (data_race(sis->flags & SWP_FS_OPS)) {
+	} else {
 		ret = swapfile_read(sis, page, synchronous);
-		goto out;
-	}
-
-	if (sis->flags & SWP_SYNCHRONOUS_IO) {
-		ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
-		if (!ret) {
-			if (trylock_page(page)) {
-				swap_slot_free_notify(page);
-				unlock_page(page);
-			}
-
-			count_vm_event(PSWPIN);
-			goto out;
-		}
 	}
-
-	ret = 0;
-	bio = bio_alloc(GFP_KERNEL, 1);
-	bio_set_dev(bio, sis->bdev);
-	bio->bi_opf = REQ_OP_READ;
-	bio->bi_iter.bi_sector = swap_page_sector(page);
-	bio->bi_end_io = end_swap_bio_read;
-	bio_add_page(bio, page, thp_size(page), 0);
-
-	disk = bio->bi_bdev->bd_disk;
-	/*
-	 * Keep this task valid during swap readpage because the oom killer may
-	 * attempt to access it in the page fault retry time check.
-	 */
-	if (synchronous) {
-		bio->bi_opf |= REQ_HIPRI;
-		get_task_struct(current);
-		bio->bi_private = current;
-	}
-	count_vm_event(PSWPIN);
-	bio_get(bio);
-	qc = submit_bio(bio);
-	while (synchronous) {
-		set_current_state(TASK_UNINTERRUPTIBLE);
-		if (!READ_ONCE(bio->bi_private))
-			break;
-
-		if (!blk_poll(disk->queue, qc, true))
-			blk_io_schedule();
-	}
-	__set_current_state(TASK_RUNNING);
-	bio_put(bio);
-
-out:
 	psi_memstall_leave(&pflags);
 	return ret;
 }