The patch titled Subject: NFS: fix nfs_release_folio() to not deadlock via kcompactd writeback has been added to the -mm mm-hotfixes-unstable branch. Its filename is nfs-fix-nfs_release_folio-to-not-deadlock-via-kcompactd-writeback.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Mike Snitzer snitzer@kernel.org Subject: NFS: fix nfs_release_folio() to not deadlock via kcompactd writeback Date: Mon, 24 Feb 2025 21:20:02 -0500

Add PF_KCOMPACTD flag and current_is_kcompactd() helper to check for it so nfs_release_folio() can skip calling nfs_wb_folio() from kcompactd.

Otherwise NFS can deadlock waiting for kcompactd enduced writeback which recurses back to NFS (which triggers writeback to NFSD via NFS loopback mount on the same host, NFSD blocks waiting for XFS's call to __filemap_get_folio):

6070.550357] INFO: task kcompactd0:58 blocked for more than 4435 seconds.

{--- [58] "kcompactd0" [<0>] folio_wait_bit+0xe8/0x200 [<0>] folio_wait_writeback+0x2b/0x80 [<0>] nfs_wb_folio+0x80/0x1b0 [nfs] [<0>] nfs_release_folio+0x68/0x130 [nfs] [<0>] split_huge_page_to_list_to_order+0x362/0x840 [<0>] migrate_pages_batch+0x43d/0xb90 [<0>] migrate_pages_sync+0x9a/0x240 [<0>] migrate_pages+0x93c/0x9f0 [<0>] compact_zone+0x8e2/0x1030 [<0>] compact_node+0xdb/0x120 [<0>] kcompactd+0x121/0x2e0 [<0>] kthread+0xcf/0x100 [<0>] ret_from_fork+0x31/0x40 [<0>] ret_from_fork_asm+0x1a/0x30 ---}

Link: https://lkml.kernel.org/r/20250225022002.26141-1-snitzer@kernel.org Fixes: 96780ca55e3cb ("NFS: fix up nfs_release_folio() to try to release the page") Signed-off-by: Mike Snitzer snitzer@kernel.org Cc: Anna Schumaker anna.schumaker@oracle.com Cc: Trond Myklebust trond.myklebust@hammerspace.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/nfs/file.c | 3 ++- include/linux/compaction.h | 5 +++++ include/linux/sched.h | 2 +- mm/compaction.c | 3 +++ 4 files changed, 11 insertions(+), 2 deletions(-)

--- a/fs/nfs/file.c~nfs-fix-nfs_release_folio-to-not-deadlock-via-kcompactd-writeback +++ a/fs/nfs/file.c @@ -29,6 +29,7 @@ #include <linux/pagemap.h> #include <linux/gfp.h> #include <linux/swap.h> +#include <linux/compaction.h>

#include <linux/uaccess.h> #include <linux/filelock.h> @@ -457,7 +458,7 @@ static bool nfs_release_folio(struct fol /* If the private flag is set, then the folio is not freeable */ if (folio_test_private(folio)) { if ((current_gfp_context(gfp) & GFP_KERNEL) != GFP_KERNEL || - current_is_kswapd()) + current_is_kswapd() || current_is_kcompactd()) return false; if (nfs_wb_folio(folio->mapping->host, folio) < 0) return false; --- a/include/linux/compaction.h~nfs-fix-nfs_release_folio-to-not-deadlock-via-kcompactd-writeback +++ a/include/linux/compaction.h @@ -80,6 +80,11 @@ static inline unsigned long compact_gap( return 2UL << order; }

+static inline int current_is_kcompactd(void) +{ + return current->flags & PF_KCOMPACTD; +} + #ifdef CONFIG_COMPACTION

extern unsigned int extfrag_for_order(struct zone *zone, unsigned int order); --- a/include/linux/sched.h~nfs-fix-nfs_release_folio-to-not-deadlock-via-kcompactd-writeback +++ a/include/linux/sched.h @@ -1701,7 +1701,7 @@ extern struct pid *cad_pid; #define PF_USED_MATH 0x00002000 /* If unset the fpu must be initialized before use */ #define PF_USER_WORKER 0x00004000 /* Kernel thread cloned from userspace thread */ #define PF_NOFREEZE 0x00008000 /* This thread should not be frozen */ -#define PF__HOLE__00010000 0x00010000 +#define PF_KCOMPACTD 0x00010000 /* I am kcompactd */ #define PF_KSWAPD 0x00020000 /* I am kswapd */ #define PF_MEMALLOC_NOFS 0x00040000 /* All allocations inherit GFP_NOFS. See memalloc_nfs_save() */ #define PF_MEMALLOC_NOIO 0x00080000 /* All allocations inherit GFP_NOIO. See memalloc_noio_save() */ --- a/mm/compaction.c~nfs-fix-nfs_release_folio-to-not-deadlock-via-kcompactd-writeback +++ a/mm/compaction.c @@ -3181,6 +3181,7 @@ static int kcompactd(void *p) long default_timeout = msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC); long timeout = default_timeout;

+ tsk->flags |= PF_KCOMPACTD; set_freezable();

pgdat->kcompactd_max_order = 0; @@ -3237,6 +3238,8 @@ static int kcompactd(void *p) pgdat->proactive_compact_trigger = false; }

+ tsk->flags &= ~PF_KCOMPACTD; + return 0; }

_

Patches currently in -mm which might be from snitzer@kernel.org are

nfs-fix-nfs_release_folio-to-not-deadlock-via-kcompactd-writeback.patch