linux-ext4 - [PATCH 1/4] fuse2fs: register as an IO flusher thread

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Message-ID: <174786679023.1385778.14264885297965601500.stgit@frogsfrogsfrogs>
Date: Wed, 21 May 2025 15:47:59 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: linux-ext4@...r.kernel.org
Subject: [PATCH 1/4] fuse2fs: register as an IO flusher thread

From: Darrick J. Wong <djwong@...nel.org>

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@...nel.org>
---
 configure       |   37 +++++++++++++++++++++++++++++++++++++
 configure.ac    |   19 +++++++++++++++++++
 lib/config.h.in |    3 +++
 misc/fuse2fs.c  |   15 +++++++++++++++
 4 files changed, 74 insertions(+)


diff --git a/configure b/configure
index f9a7aa4e2e864c..dfc6bb4a5daa2e 100755
--- a/configure
+++ b/configure
@@ -14551,6 +14551,43 @@ 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 optreset" >&5
 printf %s "checking for optreset... " >&6; }
 if test ${ac_cv_have_optreset+y}
diff --git a/configure.ac b/configure.ac
index 1f67604036b528..7f28701534a905 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1434,6 +1434,25 @@ 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 optreset exists
 dnl
diff --git a/lib/config.h.in b/lib/config.h.in
index 819c4331379247..6cd9751baab9d1 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 51f703267462b4..cbe9afd4ba1290 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
 #include <sys/ioctl.h>
 #include <unistd.h>
@@ -4792,6 +4793,20 @@ int main(int argc, char *argv[])
 		}
 	}
 
+#ifdef HAVE_PR_SET_IO_FLUSHER
+	/*
+	 * Register as a filesystem I/O server process so that our memory
+	 * allocations don't cause fs reclaim.
+	 */
+	ret = prctl(PR_SET_IO_FLUSHER, 1, 0, 0, 0);
+	if (ret < 0) {
+		err_printf(&fctx, "%s: %s.\n",
+ _("Could not register as IO flusher thread"),
+				strerror(errno));
+		ret = 0;
+	}
+#endif
+
 	/* Will we allow users to allocate every last block? */
 	if (getenv("FUSE2FS_ALLOC_ALL_BLOCKS")) {
 		log_printf(&fctx, "%s\n",