[PATCH 2/2] NFS: Improve nfsiod workqueue detection for allocation flags

[Benjamin Coddington <bcodding@redhat.com>]

The NFS client writeback paths change which flags are passed to their
memory allocation calls based on whether the current task is running from
within a workqueue or not.  More specifically, it appears that during
writeback allocations with PF_WQ_WORKER set on current->flags will add
__GFP_NORETRY | __GFP_NOWARN.  Presumably this is because nfsiod can
simply fail quickly and later retry to write back that specific page should
the allocation fail.

However, the check for PF_WQ_WORKER is too general because tasks can enter NFS
writeback paths from other workqueues.  Specifically, the loopback driver
tends to perform writeback into backing files on NFS with PF_WQ_WORKER set,
and additionally sets PF_MEMALLOC_NOIO.  The combination of
PF_MEMALLOC_NOIO with __GFP_NORETRY can easily result in allocation
failures and the loopback driver has no retry functionality.  As a result,
after commit 0bae835b63c5 ("NFS: Avoid writeback threads getting stuck in
mempool_alloc()") users are seeing corrupted loop-mounted filesystems backed
by image files on NFS.

In a preceding patch, we introduced a function to allow NFS to detect if
the task is executing within a specific workqueue.  Here we use that helper
to set __GFP_NORETRY | __GFP_NOWARN only if the workqueue is nfsiod.

Fixes: 0bae835b63c5 ("NFS: Avoid writeback threads getting stuck in mempool_alloc()")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
---
 fs/nfs/internal.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index 69c2c10ee658..173172afa3f5 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -12,6 +12,7 @@
 #include <linux/nfs_page.h>
 #include <linux/nfslocalio.h>
 #include <linux/wait_bit.h>
+#include <linux/workqueue.h>
 
 #define NFS_SB_MASK (SB_NOSUID|SB_NODEV|SB_NOEXEC|SB_SYNCHRONOUS)
 
@@ -669,9 +670,18 @@ nfs_write_match_verf(const struct nfs_writeverf *verf,
 		!nfs_write_verifier_cmp(&req->wb_verf, &verf->verifier);
 }
 
+static inline bool is_nfsiod(void)
+{
+	struct workqueue_struct *current_wq = current_workqueue();
+
+	if (current_wq)
+		return current_wq == nfsiod_workqueue;
+	return false;
+}
+
 static inline gfp_t nfs_io_gfp_mask(void)
 {
-	if (current->flags & PF_WQ_WORKER)
+	if (is_nfsiod())
 		return GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
 	return GFP_KERNEL;
 }
-- 
2.47.0