[RFC PATCH 1/1] mm/filemap: make writeback wait killable in __filemap_fdatawait_range()

[Rushil Patel <rushil.patel@gsacapital.com>]

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__filemap_fdatawait_range() waits for writeback using
folio_wait_writeback(), which sleeps in TASK_UNINTERRUPTIBLE. On network
filesystems (NFS, CIFS, 9P) the server can stall for minutes or hours,
leaving processes in unkillable D-state. The only recovery is a node
reboot.

Replace folio_wait_writeback() with folio_wait_writeback_killable() so
that SIGKILL can interrupt the wait. On fatal signal, release the folio
batch and return early. The writeback itself continues on the server --
only the client-side wait is interrupted. The dying process never
inspects the return value, and other processes get correct error
reporting through errseq_t (mapping->wb_err) which is set by the
writeback completion path independently of this wait.

Additionally, skip the writeback wait entirely when the calling process
has PF_EXITING set. This handles a re-entry trap discovered in testing:
when SIGKILL wakes a process from the killable wait, do_exit() sets
PF_EXITING and then exit_files() closes file descriptors, which on NFS
triggers nfs4_file_flush() -> nfs_wb_all() -> filemap_write_and_wait()
-> __filemap_fdatawait_range(). At that point wants_signal()
(kernel/signal.c) rejects all signals for PF_EXITING tasks -- including
SIGKILL -- because the PF_EXITING check precedes the SIGKILL exception.
The second killable wait would therefore block forever. Checking
PF_EXITING at entry avoids this: the process is dying and has no use for
writeback confirmation.

Signed-off-by: Rushil Patel <rushil.patel@gsacapital.com>
---
 mm/filemap.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 406cef06b684..d348f1dd75f9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -515,6 +515,15 @@ static void __filemap_fdatawait_range(struct address_space *mapping,
 	struct folio_batch fbatch;
 	unsigned nr_folios;
 
+	/*
+	 * If the process is exiting (PF_EXITING), skip the writeback wait.
+	 * During do_exit(), nfs4_file_flush() re-enters this function, but
+	 * wants_signal() rejects signals for PF_EXITING tasks so a second
+	 * SIGKILL cannot wake us from the TASK_KILLABLE wait below.
+	 */
+	if (current->flags & PF_EXITING)
+		return;
+
 	folio_batch_init(&fbatch);
 
 	while (index <= end) {
@@ -529,7 +538,10 @@ static void __filemap_fdatawait_range(struct address_space *mapping,
 		for (i = 0; i < nr_folios; i++) {
 			struct folio *folio = fbatch.folios[i];
 
-			folio_wait_writeback(folio);
+			if (folio_wait_writeback_killable(folio)) {
+				folio_batch_release(&fbatch);
+				return;
+			}
 		}
 		folio_batch_release(&fbatch);
 		cond_resched();
-- 
2.47.3

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