Re: [syzbot] [netfs?] WARNING in netfs_writepages (3)

[David Howells]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

commit 6836db8d3babf7ae82597d516f89848e5652b957
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Fri Jul 24 10:24:30 2026 +0100

    netfs: Fix folio_queue ENOMEM in writeback by adding a mempool
    
    Fix the handling of folio_queue allocation failure in writeback by adding a
    mempool and passing in gfp_t flags to the rolling buffer functions that
    allocate memory, using the mempool if gfp != GFP_KERNEL.
    
    This is then extended upwards and the gfp to be used for a request is stored
    in the netfs_io_request struct and is then used for both requests and
    subrequests, eliminating the sleeping loops there.
    
    The failure caused:
    
        folio != NULL
        WARNING: fs/netfs/write_issue.c:603 at netfs_writepages+0x883/0xa10 fs/netfs/write_issue.c:603, CPU#3: syz.0.17/5919
    
    Fixes: cd0277ed0c18 ("netfs: Use new folio_queue data type and iterator instead of xarray iter")
    Reported-by: syzbot+0da43efa72f88bd3a8af@xxxxxxxxxxxxxxxxxxxxxxxxx
    Closes: https://syzkaller.appspot.com/bug?extid=0da43efa72f88bd3a8af
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
    Tested-by: syzbot+0da43efa72f88bd3a8af@xxxxxxxxxxxxxxxxxxxxxxxxx
    cc: Paulo Alcantara <pc@xxxxxxxxxxxxx>
    cc: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
    cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
    cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
    cc: netfs@xxxxxxxxxxxxxxx
    cc: linux-fsdevel@xxxxxxxxxxxxxxx

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index 3d86414ee40f..7fdfa4f27e34 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -361,7 +361,7 @@ void netfs_readahead(struct readahead_control *ractl)
 	netfs_rreq_expand(rreq, ractl);
 
 	rreq->submitted = rreq->start;
-	if (rolling_buffer_init(&rreq->buffer, rreq->debug_id, ITER_DEST) < 0)
+	if (rolling_buffer_init(&rreq->buffer, rreq->debug_id, ITER_DEST, rreq->gfp) < 0)
 		goto cleanup_free;
 	netfs_read_to_pagecache(rreq, ractl);
 
@@ -380,10 +380,10 @@ static int netfs_create_singular_buffer(struct netfs_io_request *rreq, struct fo
 {
 	ssize_t added;
 
-	if (rolling_buffer_init(&rreq->buffer, rreq->debug_id, ITER_DEST) < 0)
+	if (rolling_buffer_init(&rreq->buffer, rreq->debug_id, ITER_DEST, rreq->gfp) < 0)
 		return -ENOMEM;
 
-	added = rolling_buffer_append(&rreq->buffer, folio, rollbuf_flags);
+	added = rolling_buffer_append(&rreq->buffer, folio, rollbuf_flags, rreq->gfp);
 	if (added < 0)
 		return added;
 	rreq->submitted = rreq->start + added;
diff --git a/fs/netfs/internal.h b/fs/netfs/internal.h
index d889caa401dc..420ee7b26580 100644
--- a/fs/netfs/internal.h
+++ b/fs/netfs/internal.h
@@ -43,6 +43,7 @@ extern struct list_head netfs_io_requests;
 extern spinlock_t netfs_proc_lock;
 extern mempool_t netfs_request_pool;
 extern mempool_t netfs_subrequest_pool;
+extern mempool_t netfs_folioq_pool;
 
 #ifdef CONFIG_PROC_FS
 static inline void netfs_proc_add_rreq(struct netfs_io_request *rreq)
diff --git a/fs/netfs/main.c b/fs/netfs/main.c
index 73da6c9f5777..927badf3989d 100644
--- a/fs/netfs/main.c
+++ b/fs/netfs/main.c
@@ -28,6 +28,7 @@ static struct kmem_cache *netfs_request_slab;
 static struct kmem_cache *netfs_subrequest_slab;
 mempool_t netfs_request_pool;
 mempool_t netfs_subrequest_pool;
+mempool_t netfs_folioq_pool;
 
 #ifdef CONFIG_PROC_FS
 LIST_HEAD(netfs_io_requests);
@@ -108,6 +109,9 @@ static int __init netfs_init(void)
 {
 	int ret = -ENOMEM;
 
+	if (mempool_init_kmalloc_pool(&netfs_folioq_pool, 100, sizeof(struct folio_queue)) < 0)
+		goto error_folioq_pool;
+
 	netfs_request_slab = kmem_cache_create("netfs_request",
 					       sizeof(struct netfs_io_request), 0,
 					       SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT,
@@ -160,6 +164,8 @@ static int __init netfs_init(void)
 error_reqpool:
 	kmem_cache_destroy(netfs_request_slab);
 error_req:
+	mempool_exit(&netfs_folioq_pool);
+error_folioq_pool:
 	return ret;
 }
 fs_initcall(netfs_init);
@@ -172,5 +178,6 @@ static void __exit netfs_exit(void)
 	kmem_cache_destroy(netfs_subrequest_slab);
 	mempool_exit(&netfs_request_pool);
 	kmem_cache_destroy(netfs_request_slab);
+	mempool_exit(&netfs_folioq_pool);
 }
 module_exit(netfs_exit);
diff --git a/fs/netfs/objects.c b/fs/netfs/objects.c
index b8c4918d3dcd..29d8eab06dec 100644
--- a/fs/netfs/objects.c
+++ b/fs/netfs/objects.c
@@ -7,7 +7,6 @@
 
 #include <linux/slab.h>
 #include <linux/mempool.h>
-#include <linux/delay.h>
 #include "internal.h"
 
 static void netfs_free_request(struct work_struct *work);
@@ -26,17 +25,23 @@ struct netfs_io_request *netfs_alloc_request(struct address_space *mapping,
 	struct netfs_io_request *rreq;
 	mempool_t *mempool = ctx->ops->request_pool ?: &netfs_request_pool;
 	struct kmem_cache *cache = mempool->pool_data;
+	gfp_t gfp = GFP_KERNEL;
 	int ret;
 
-	for (;;) {
-		rreq = mempool_alloc(mempool, GFP_KERNEL);
-		if (rreq)
-			break;
-		msleep(10);
+	/* Writeback is part of memory reclaim and must not fail due to ENOMEM. */
+	if (origin == NETFS_WRITEBACK || origin == NETFS_WRITEBACK_SINGLE) {
+		gfp = GFP_NOFS; /* Allows use of mempools. */
+
+		rreq = mempool_alloc(mempool, gfp);
+	} else {
+		rreq = mempool->alloc(gfp, mempool->pool_data);
+		if (!rreq)
+			return ERR_PTR(-ENOMEM);
 	}
 
 	memset(rreq, 0, kmem_cache_size(cache));
 	INIT_WORK(&rreq->cleanup_work, netfs_free_request);
+	rreq->gfp	= gfp;
 	rreq->start	= start;
 	rreq->len	= len;
 	rreq->origin	= origin;
@@ -200,12 +205,14 @@ struct netfs_io_subrequest *netfs_alloc_subrequest(struct netfs_io_request *rreq
 	mempool_t *mempool = rreq->netfs_ops->subrequest_pool ?: &netfs_subrequest_pool;
 	struct kmem_cache *cache = mempool->pool_data;
 
-	for (;;) {
-		subreq = mempool_alloc(rreq->netfs_ops->subrequest_pool ?: &netfs_subrequest_pool,
-				       GFP_KERNEL);
-		if (subreq)
-			break;
-		msleep(10);
+	if (rreq->gfp == GFP_KERNEL) {
+		subreq = mempool->alloc(rreq->gfp, mempool->pool_data);
+		if (!subreq)
+			return ERR_PTR(-ENOMEM);
+	} else {
+		subreq = mempool_alloc(mempool, rreq->gfp);
+		if (!subreq)
+			return NULL;
 	}
 
 	memset(subreq, 0, kmem_cache_size(cache));
diff --git a/fs/netfs/read_pgpriv2.c b/fs/netfs/read_pgpriv2.c
index 7eacc58abadb..c31190993b76 100644
--- a/fs/netfs/read_pgpriv2.c
+++ b/fs/netfs/read_pgpriv2.c
@@ -53,7 +53,7 @@ static void netfs_pgpriv2_copy_folio(struct netfs_io_request *creq, struct folio
 	trace_netfs_folio(folio, netfs_folio_trace_store_copy);
 
 	/* Attach the folio to the rolling buffer. */
-	if (rolling_buffer_append(&creq->buffer, folio, 0) < 0) {
+	if (rolling_buffer_append(&creq->buffer, folio, 0, creq->gfp) < 0) {
 		folio_end_private_2(folio);
 		clear_bit(NETFS_RREQ_FOLIO_COPY_TO_CACHE, &creq->flags);
 		return;
diff --git a/fs/netfs/rolling_buffer.c b/fs/netfs/rolling_buffer.c
index a17fbf9853a4..c7f107764bda 100644
--- a/fs/netfs/rolling_buffer.c
+++ b/fs/netfs/rolling_buffer.c
@@ -27,7 +27,10 @@ struct folio_queue *netfs_folioq_alloc(unsigned int rreq_id, gfp_t gfp,
 {
 	struct folio_queue *fq;
 
-	fq = kmalloc_obj(*fq, gfp);
+	if (gfp == GFP_KERNEL)
+		fq = netfs_folioq_pool.alloc(gfp, netfs_folioq_pool.pool_data);
+	else
+		fq = mempool_alloc(&netfs_folioq_pool, gfp);
 	if (fq) {
 		netfs_stat(&netfs_n_folioq);
 		folioq_init(fq, rreq_id);
@@ -50,7 +53,7 @@ void netfs_folioq_free(struct folio_queue *folioq,
 {
 	trace_netfs_folioq(folioq, trace);
 	netfs_stat_d(&netfs_n_folioq);
-	kfree(folioq);
+	mempool_free(folioq, &netfs_folioq_pool);
 }
 EXPORT_SYMBOL(netfs_folioq_free);
 
@@ -60,11 +63,11 @@ EXPORT_SYMBOL(netfs_folioq_free);
  * consumer.
  */
 int rolling_buffer_init(struct rolling_buffer *roll, unsigned int rreq_id,
-			unsigned int direction)
+			unsigned int direction, gfp_t gfp)
 {
 	struct folio_queue *fq;
 
-	fq = netfs_folioq_alloc(rreq_id, GFP_NOFS, netfs_trace_folioq_rollbuf_init);
+	fq = netfs_folioq_alloc(rreq_id, gfp, netfs_trace_folioq_rollbuf_init);
 	if (!fq)
 		return -ENOMEM;
 
@@ -77,14 +80,14 @@ int rolling_buffer_init(struct rolling_buffer *roll, unsigned int rreq_id,
 /*
  * Add another folio_queue to a rolling buffer if there's no space left.
  */
-int rolling_buffer_make_space(struct rolling_buffer *roll)
+int rolling_buffer_make_space(struct rolling_buffer *roll, gfp_t gfp)
 {
 	struct folio_queue *fq, *head = roll->head;
 
 	if (!folioq_full(head))
 		return 0;
 
-	fq = netfs_folioq_alloc(head->rreq_id, GFP_NOFS, netfs_trace_folioq_make_space);
+	fq = netfs_folioq_alloc(head->rreq_id, gfp, netfs_trace_folioq_make_space);
 	if (!fq)
 		return -ENOMEM;
 	fq->prev = head;
@@ -122,7 +125,7 @@ ssize_t rolling_buffer_load_from_ra(struct rolling_buffer *roll,
 	int nr, ix, to;
 	ssize_t size = 0;
 
-	if (rolling_buffer_make_space(roll) < 0)
+	if (rolling_buffer_make_space(roll, GFP_KERNEL) < 0)
 		return -ENOMEM;
 
 	fq = roll->head;
@@ -153,12 +156,12 @@ ssize_t rolling_buffer_load_from_ra(struct rolling_buffer *roll,
  * Append a folio to the rolling buffer.
  */
 ssize_t rolling_buffer_append(struct rolling_buffer *roll, struct folio *folio,
-			      unsigned int flags)
+			      unsigned int flags, gfp_t gfp)
 {
 	ssize_t size = folio_size(folio);
 	int slot;
 
-	if (rolling_buffer_make_space(roll) < 0)
+	if (rolling_buffer_make_space(roll, gfp) < 0)
 		return -ENOMEM;
 
 	slot = folioq_append(roll->head, folio);
diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index 14efe4cb9393..2d9cfcd43658 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -108,7 +108,7 @@ struct netfs_io_request *netfs_create_write_req(struct address_space *mapping,
 	ictx = netfs_inode(wreq->inode);
 	if (is_cacheable)
 		fscache_begin_write_operation(&wreq->cache_resources, netfs_i_cookie(ictx));
-	if (rolling_buffer_init(&wreq->buffer, wreq->debug_id, ITER_SOURCE) < 0)
+	if (rolling_buffer_init(&wreq->buffer, wreq->debug_id, ITER_SOURCE, wreq->gfp) < 0)
 		goto nomem;
 
 	wreq->cleaned_to = wreq->start;
@@ -167,7 +167,7 @@ void netfs_prepare_write(struct netfs_io_request *wreq,
 	 */
 	if (iov_iter_is_folioq(wreq_iter) &&
 	    wreq_iter->folioq_slot >= folioq_nr_slots(wreq_iter->folioq))
-		rolling_buffer_make_space(&wreq->buffer);
+		rolling_buffer_make_space(&wreq->buffer, wreq->gfp);
 
 	subreq = netfs_alloc_subrequest(wreq);
 	subreq->source		= stream->source;
@@ -334,7 +334,7 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 
 	_enter("");
 
-	if (rolling_buffer_make_space(&wreq->buffer) < 0)
+	if (rolling_buffer_make_space(&wreq->buffer, wreq->gfp) < 0)
 		return -ENOMEM;
 
 	/* netfs_perform_write() may shift i_size around the page or from out
@@ -436,7 +436,7 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 	}
 
 	/* Attach the folio to the rolling buffer. */
-	rolling_buffer_append(&wreq->buffer, folio, 0);
+	rolling_buffer_append(&wreq->buffer, folio, 0, wreq->gfp);
 
 	/* Move the submission point forward to allow for write-streaming data
 	 * not starting at the front of the page.  We don't do write-streaming
@@ -749,7 +749,7 @@ static int netfs_write_folio_single(struct netfs_io_request *wreq,
 
 	/* Attach the folio to the rolling buffer. */
 	folio_get(folio);
-	ret = rolling_buffer_append(&wreq->buffer, folio, NETFS_ROLLBUF_PUT_MARK);
+	ret = rolling_buffer_append(&wreq->buffer, folio, NETFS_ROLLBUF_PUT_MARK, wreq->gfp);
 	if (ret < 0) {
 		folio_put(folio);
 		return ret;
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 1bc120d61c5b..d0b62d53eea9 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -255,6 +255,7 @@ struct netfs_io_request {
 	unsigned long long	cleaned_to;	/* Position we've cleaned folios to */
 	unsigned long long	abandon_to;	/* Position to abandon folios to */
 	const struct folio	*no_unlock_folio; /* Don't unlock this folio after read */
+	gfp_t			gfp;		/* GFP flags to use */
 	unsigned int		direct_bv_count; /* Number of elements in direct_bv[] */
 	unsigned int		debug_id;
 	unsigned int		rsize;		/* Maximum read size (0 for none) */
diff --git a/include/linux/rolling_buffer.h b/include/linux/rolling_buffer.h
index ac15b1ffdd83..9e5dad29669c 100644
--- a/include/linux/rolling_buffer.h
+++ b/include/linux/rolling_buffer.h
@@ -43,13 +43,13 @@ struct rolling_buffer_snapshot {
 #define ROLLBUF_MARK_2	BIT(1)
 
 int rolling_buffer_init(struct rolling_buffer *roll, unsigned int rreq_id,
-			unsigned int direction);
-int rolling_buffer_make_space(struct rolling_buffer *roll);
+			unsigned int direction, gfp_t gfp);
+int rolling_buffer_make_space(struct rolling_buffer *roll, gfp_t gfp);
 ssize_t rolling_buffer_load_from_ra(struct rolling_buffer *roll,
 				    struct readahead_control *ractl,
 				    struct folio_batch *put_batch);
 ssize_t rolling_buffer_append(struct rolling_buffer *roll, struct folio *folio,
-			      unsigned int flags);
+			      unsigned int flags, gfp_t gfp);
 struct folio_queue *rolling_buffer_delete_spent(struct rolling_buffer *roll);
 void rolling_buffer_clear(struct rolling_buffer *roll);