[PATCH v4] Btrfs: fix deadlock with memory reclaim during scrub

[fdmanana@kernel.org]

From: Filipe Manana <fdmanana@suse.com>

When a transaction commit starts, it attempts to pause scrub and it blocks
until the scrub is paused. So while the transaction is blocked waiting for
scrub to pause, we can not do memory allocation with GFP_KERNEL from scrub,
otherwise we risk getting into a deadlock with reclaim.

Checking for scrub pause requests is done early at the beginning of the
while loop of scrub_stripe() and later in the loop, scrub_extent() and
scrub_raid56_parity() are called, which in turn call scrub_pages() and
scrub_pages_for_parity() respectively. These last two functions do memory
allocations using GFP_KERNEL. Same problem could happen while scrubbing
the super blocks, since it calls scrub_pages().

So make sure GFP_NOFS is used for the memory allocations because at any
time a scrub pause request can happen from another task that started to
commit a transaction.

Fixes: 58c4e173847a ("btrfs: scrub: use GFP_KERNEL on the submission path")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
---

V2: Make using GFP_NOFS unconditionial. Previous version was racy, as pausing
requests migth happen just after we checked for them.

V3: Use memalloc_nofs_save() just like V1 did.

V4: Similar problem happened for raid56, which was previously missed, so
    deal with it as well as the case for scrub_supers().

 fs/btrfs/scrub.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 3be1456b5116..e08b7502d1f0 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3779,6 +3779,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	struct scrub_ctx *sctx;
 	int ret;
 	struct btrfs_device *dev;
+	unsigned int nofs_flag;
 
 	if (btrfs_fs_closing(fs_info))
 		return -EINVAL;
@@ -3882,6 +3883,16 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	atomic_inc(&fs_info->scrubs_running);
 	mutex_unlock(&fs_info->scrub_lock);
 
+	/*
+	 * In order to avoid deadlock with reclaim when there is a transaction
+	 * trying to pause scrub, make sure we use GFP_NOFS for all the
+	 * allocations done at btrfs_scrub_pages() and scrub_pages_for_parity()
+	 * invoked by our callees. The pausing request is done when the
+	 * transaction commit starts, and it blocks the transaction until scrub
+	 * is paused (done at specific points at scrub_stripe() or right above
+	 * before incrementing fs_info->scrubs_running).
+	 */
+	nofs_flag = memalloc_nofs_save();
 	if (!is_dev_replace) {
 		/*
 		 * by holding device list mutex, we can
@@ -3895,6 +3906,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	if (!ret)
 		ret = scrub_enumerate_chunks(sctx, dev, start, end,
 					     is_dev_replace);
+	memalloc_nofs_restore(nofs_flag);
 
 	wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);
 	atomic_dec(&fs_info->scrubs_running);
-- 
2.11.0