[PATCH 2/3] fuse2fs: register as an IO flusher thread

Darrick J. Wong
2025-11-06T22:39:52Z

fuse2fs is involved in the filesystem I/O path and can allocate memory
while processing I/O requests. Therefore, we need to register that fact
with the kernel so that memory allocations done by libext2fs don't start
a round of filesystem memory reclaim, which could cause more writeout to
get dumped on fuse2fs.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
---
configure | 37 +++++++++++++++++++++++++++++++++++++
configure.ac | 19 +++++++++++++++++++
lib/config.h.in | 3 +++
misc/fuse2fs.c | 27 +++++++++++++++++++++++++++
4 files changed, 86 insertions(+)

diff --git a/configure b/configure
index ba2e5380f6d20b..356644beed651e 100755
--- a/configure
+++ b/configure
@@ -14725,6 +14725,43 @@ then
printf "%s\n" "#define FUSE_USE_VERSION $FUSE_USE_VERSION" >>confdefs.h

fi
+
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for PR_SET_IO_FLUSHER" >&5
+printf %s "checking for PR_SET_IO_FLUSHER... " >&6; }
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h. */
+
+#define _GNU_SOURCE
+#include <sys/prctl.h>
+
+int
+main (void)
+{
+
+prctl(PR_SET_IO_FLUSHER, 0, 0, 0, 0);
+
+ ;
+ return 0;
+}
+
+_ACEOF
+if ac_fn_c_try_link "$LINENO"
+then :
+ have_pr_set_io_flusher=yes
+ { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+printf "%s\n" "yes" >&6; }
+else $as_nop
+ { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: no" >&5
+printf "%s\n" "no" >&6; }
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.beam \
+ conftest$ac_exeext conftest.$ac_ext
+if test "$have_pr_set_io_flusher" = yes; then
+
+printf "%s\n" "#define HAVE_PR_SET_IO_FLUSHER 1" >>confdefs.h
+
+fi
+
{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for CLOCK_MONOTONIC" >&5
printf %s "checking for CLOCK_MONOTONIC... " >&6; }
cat confdefs.h - <<_ACEOF >conftest.$ac_ext
diff --git a/configure.ac b/configure.ac
index 051161c5848072..f065cd395cf33c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1452,6 +1452,25 @@ then
AC_DEFINE_UNQUOTED(FUSE_USE_VERSION, $FUSE_USE_VERSION,
[Define to the version of FUSE to use])
fi
+
+dnl
+dnl see if PR_SET_IO_FLUSHER exists
+dnl
+AC_MSG_CHECKING(for PR_SET_IO_FLUSHER)
+AC_LINK_IFELSE(
+[ AC_LANG_PROGRAM([[
+#define _GNU_SOURCE
+#include <sys/prctl.h>
+ ]], [[
+prctl(PR_SET_IO_FLUSHER, 0, 0, 0, 0);
+ ]])
+], have_pr_set_io_flusher=yes
+ AC_MSG_RESULT(yes),
+ AC_MSG_RESULT(no))
+if test "$have_pr_set_io_flusher" = yes; then
+ AC_DEFINE(HAVE_PR_SET_IO_FLUSHER, 1, [Define to 1 if PR_SET_IO_FLUSHER is present])
+fi
+
dnl
dnl see if CLOCK_MONOTONIC exists
dnl
diff --git a/lib/config.h.in b/lib/config.h.in
index 364d6e865b7115..a4d8ce1c3765ed 100644
--- a/lib/config.h.in
+++ b/lib/config.h.in
@@ -70,6 +70,9 @@
/* Define to 1 if you have the BSD-style 'qsort_r' function. */
#undef HAVE_BSD_QSORT_R

+/* Define to 1 if PR_SET_IO_FLUSHER is present */
+#undef HAVE_PR_SET_IO_FLUSHER
+
/* Define to 1 if you have the Mac OS X function
CFLocaleCopyPreferredLanguages in the CoreFoundation framework. */
#undef HAVE_CFLOCALECOPYPREFERREDLANGUAGES
diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index e0134834d342dd..86e0222a51369a 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -17,6 +17,7 @@
# include <linux/fs.h>
# include <linux/falloc.h>
# include <linux/xattr.h>
+# include <sys/prctl.h>
#endif
#ifdef HAVE_SYS_XATTR_H
#include <sys/xattr.h>
@@ -5483,6 +5484,30 @@ static void fuse2fs_compute_libfuse_args(struct fuse2fs *ff,
}
}

+/*
+ * Try to register as a filesystem I/O server process so that our memory
+ * allocations don't cause fs reclaim.
+ */
+static void try_set_io_flusher(struct fuse2fs *ff)
+{
+#ifdef HAVE_PR_SET_IO_FLUSHER
+ int ret = prctl(PR_GET_IO_FLUSHER, 0, 0, 0, 0);
+
+ /*
+ * positive ret means it's already set, negative means we can't even
+ * look at the value so don't bother setting it
+ */
+ if (ret)
+ return;
+
+ ret = prctl(PR_SET_IO_FLUSHER, 1, 0, 0, 0);
+ if (ret < 0)
+ err_printf(ff, "%s: %s.\n",
+ _("Could not register as IO flusher thread"),
+ strerror(errno));
+#endif
+}
+
int main(int argc, char *argv[])
{
struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
@@ -5528,6 +5553,8 @@ int main(int argc, char *argv[])
goto out;
}

+ try_set_io_flusher(&fctx);
+
/* Will we allow users to allocate every last block? */
if (getenv("FUSE2FS_ALLOC_ALL_BLOCKS")) {
log_printf(&fctx, "%s\n",