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

David Howells
2021-08-12T20:22:56Z

[!] 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;
}