Support Linux v6.13 FUSE max_page_limit

libfuse/include/fuse_common.h

@@ -37,7 +37,7 @@
 #endif
 
 #define FUSE_DEFAULT_MAX_PAGES_PER_REQ 32
-#define FUSE_MAX_MAX_PAGES 256
+#define FUSE_DEFAULT_MAX_MAX_PAGES 256
 
 EXTERN_C_BEGIN
 

libfuse/lib/fuse_msgbuf.cpp

@@ -73,7 +73,7 @@ msgbuf_constructor()
 {
   g_PAGESIZE = sysconf(_SC_PAGESIZE);
   // FUSE_MAX_MAX_PAGES for payload + 1 for message header
-  msgbuf_set_bufsize(FUSE_MAX_MAX_PAGES + 1);
+  msgbuf_set_bufsize(FUSE_DEFAULT_MAX_MAX_PAGES + 1);
 }
 
 static

libfuse/lib/helper.c

@@ -258,7 +258,7 @@ fuse_mount_common(const char       *mountpoint_,
     return NULL;
 
   pagesize = sysconf(_SC_PAGESIZE);
-  bufsize  = ((FUSE_MAX_MAX_PAGES + 1) * pagesize);
+  bufsize  = ((FUSE_DEFAULT_MAX_MAX_PAGES + 1) * pagesize);
 
   ch = fuse_chan_new(fd,bufsize);
   if(!ch)

mkdocs/docs/config/fuse_msg_size.md

@@ -1,7 +1,8 @@
 # fuse_msg_size
 
-* `fuse_msg_size=UINT`
-* Defaults to `256`
+* `fuse_msg_size=UINT|SIZE`
+* Defaults to `1M`
+* Performance improvements often peak at about `4M`
 
 FUSE applications communicate with the kernel over a special character
 device: `/dev/fuse`. A large portion of the overhead associated with
@@ -16,14 +17,33 @@ halved.
 In Linux v4.20 a new feature was added allowing the negotiation of the
 max message size. Since the size is in multiples of
 [pages](https://en.wikipedia.org/wiki/Page_(computer_memory)) the
-feature is called `max_pages`. There is a maximum `max_pages` value of
-256 (1MiB) and minimum of 1 (4KiB). The default used by Linux >=4.20,
-and hardcoded value used before 4.20, is 32 (128KiB). In mergerfs it's
-referred to as fuse_msg_size to make it clear what it impacts and
-provide some abstraction.
+feature is called `max_pages`. In versions of Linux prior to v6.13
+there is a maximum `max_pages` value of 256 (1MiB) and minimum of 1
+(4KiB). In [Linux
+v6.13](https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2b3933b1e0a0a4b758fbc164bb31db0c113a7e2c)
+and above the max value supported by the kernel can range from 1
+(4KiB) to 65535 (~256MiB) (assuming a page size of 4KiB.) The default
+used by Linux >= 4.20, and hard coded value used before 4.20, is 32
+(128KiB). In mergerfs it is referred to as `fuse_msg_size` to make it
+clear what it impacts and provide some abstraction.
 
-Since there should be no downsides to increasing `fuse_msg_size`,
-outside a minor increase in RAM usage due to larger message buffers,
-mergerfs defaults the value to 256. On kernels before v4.20 the value
-has no effect. The reason the value is configurable is to enable
-experimentation and benchmarking.
+If the `fuse_msg_size` value provided is more than the system wide
+maximum mergerfs will attempt to increase the system wide value to keep
+the user from needing to set the value using `sysctl`,
+`/etc/sysctl.conf`, or `/proc/sys/fs/fuse/max_pages_limit`.
+
+The main downside to increasing the value is that memory usage will
+increase approximately relative to the number of [processing
+threads](threads.md) configured. Keep this in mind for systems with
+lower amounts of memory like most SBCs. Performance improvements seem
+to peak around 4MiB.
+
+On kernels before v4.20 the option has no effect. On kernels between
+v4.20 and v6.13 the max value is 256. On kernels >= v6.13 the maximum
+value is 65535.
+
+Since page size can differ between systems mergerfs can take a value in
+bytes and will convert it to the proper number of pages (rounded up).
+
+NOTE: If you intend to enable `cache.files` you should also set
+[readahead](readahead.md) to match `fuse_msg_size`.

mkdocs/docs/config/options.md

@@ -8,7 +8,9 @@ These options are the same regardless of whether you use them with the
 - BOOL = 'true' | 'false'
 - INT = [MIN_INT,MAX_INT]
 - UINT = [0,MAX_INT]
-- SIZE = 'NNM'; NN = INT, M = 'K' | 'M' | 'G' | 'T'
+- SIZE = 'NNM'; NN = INT, M = 'B' | 'K' | 'M' | 'G' | 'T'
+- PAGESIZE = UINT (representing number of pages) | SIZE (in bytes
+  which will be converted to pages)
 - STR = string (may refer to an enumerated value, see details of
   argument)
 - FUNC = filesystem function
@@ -109,9 +111,9 @@ These options are the same regardless of whether you use them with the
   unavailable the kernel will ensure there is at most one pending read
   request per file handle and will attempt to order requests by
   offset. (default: true)
-- **[fuse_msg_size](fuse_msg_size.md)=UINT**: Set the max number of
+- **[fuse_msg_size](fuse_msg_size.md)=PAGESIZE**: Set the max number of
   pages per FUSE message. Only available on Linux >= 4.20 and ignored
-  otherwise. (min: 1; max: 256; default: 256)
+  otherwise. (min: 1; max: 65535; default: "1M")
 - **[threads](threads.md)=INT**: Number of threads to use. When used
   alone (`process-thread-count=-1`) it sets the number of threads
   reading and processing FUSE messages. When used together it sets the

mkdocs/docs/config/readahead.md

@@ -11,8 +11,9 @@ doesn't mean that is the size used by the kernel for read and
 writes.
 
 Linux has a max read/write size of 2GB. Since the max FUSE message
-size is just over 1MB the kernel will break up read and write requests
-with buffers larger than that 1MB.
+size is just over 1MiB (by default on more recent kernels) the kernel
+will break up read and write requests with buffers larger than that
+1MiB.
 
 When page caching is disabled (`cache.files=off`), besides the kernel
 breaking up requests with larger buffers, requests are effectively one
@@ -35,4 +36,5 @@ a generic feature but there is no standard way to do so mergerfs added
 this feature to make it easier to set.
 
 There is currently no way to set separate values for different
-branches through mergerfs.
+branches through mergerfs. In fact at some point the feature may be
+changed to only set mergerfs' readahead.

src/config.cpp

@@ -102,7 +102,7 @@ Config::Config()
     follow_symlinks(FollowSymlinks::ENUM::NEVER),
     fsname(),
     func(),
-    fuse_msg_size(FUSE_MAX_MAX_PAGES),
+    fuse_msg_size("1M"),
     ignorepponrename(false),
     inodecalc("hybrid-hash"),
     lazy_umount_mountpoint(false),

src/config.hpp

@@ -21,12 +21,13 @@
 #include "config_cachefiles.hpp"
 #include "config_flushonclose.hpp"
 #include "config_follow_symlinks.hpp"
-#include "config_pid.hpp"
 #include "config_inodecalc.hpp"
 #include "config_link_exdev.hpp"
 #include "config_log_metrics.hpp"
 #include "config_moveonenospc.hpp"
 #include "config_nfsopenhack.hpp"
+#include "config_pagesize.hpp"
+#include "config_pid.hpp"
 #include "config_rename_exdev.hpp"
 #include "config_set.hpp"
 #include "config_statfs.hpp"
@@ -124,7 +125,7 @@ public:
   FollowSymlinks follow_symlinks;
   ConfigSTR      fsname;
   Funcs          func;
-  ConfigUINT64   fuse_msg_size;
+  ConfigPageSize fuse_msg_size;
   ConfigBOOL     ignorepponrename;
   InodeCalc      inodecalc;
   ConfigBOOL     kernel_cache;

src/config_pagesize.cpp

@@ -0,0 +1,61 @@
+/*
+  ISC License
+
+  Copyright (c) 2025, Antonio SJ Musumeci <trapexit@spawn.link>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#include "config_pagesize.hpp"
+
+#include "from_string.hpp"
+
+#include <cctype>
+#include <string>
+
+#include <unistd.h>
+
+
+ConfigPageSize::ConfigPageSize(const uint64_t v_)
+  : _v(v_)
+{
+
+}
+
+ConfigPageSize::ConfigPageSize(const std::string &s_)
+{
+  from_string(s_);
+}
+
+std::string
+ConfigPageSize::to_string(void) const
+{
+  return std::to_string(_v);
+}
+
+int
+ConfigPageSize::from_string(const std::string &s_)
+{
+  uint64_t v;
+  uint64_t pagesize;
+
+  pagesize = sysconf(_SC_PAGESIZE);
+
+  str::from(s_,&v);
+  if(!std::isalpha(s_.back()))
+    v *= pagesize;
+
+  _v = ((v + pagesize - 1) / pagesize);
+
+  return 0;
+}

src/config_pagesize.hpp

@@ -0,0 +1,51 @@
+/*
+  ISC License
+
+  Copyright (c) 2025, Antonio SJ Musumeci <trapexit@spawn.link>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#pragma once
+
+#include "tofrom_string.hpp"
+
+#include <cstdint>
+
+class ConfigPageSize : public ToFromString
+{
+private:
+  uint64_t _v;
+
+public:
+  ConfigPageSize(const uint64_t);
+  ConfigPageSize(const std::string &);
+
+public:
+  std::string to_string(void) const final;
+  int from_string(const std::string &) final;
+
+public:
+  ConfigPageSize&
+  operator=(const uint64_t v_)
+  {
+    _v = v_;
+
+    return *this;
+  }
+
+  operator const uint64_t() const
+  {
+    return _v;
+  }
+};

src/from_string.cpp

@@ -76,6 +76,11 @@ namespace str
     tmp = ::strtoll(value_.c_str(),&endptr,10);
     switch(*endptr)
       {
+      case 'b':
+      case 'B':
+        tmp *= 1ULL;
+        break;
+
       case 'k':
       case 'K':
         tmp *= 1024ULL;

src/fuse_init.cpp

@@ -23,7 +23,10 @@
 
 #include "fuse.h"
 
+#include "ghc/filesystem.hpp"
+
 #include <thread>
+#include <algorithm>
 
 
 namespace l
@@ -68,15 +71,67 @@ namespace l
     *want_ = false;
   }
 
+  #define MAX_FUSE_MSG_SIZE 65535
+  static const char MAX_PAGES_LIMIT_FILEPATH[] = "/proc/sys/fs/fuse/max_pages_limit";
+
   static
   void
   want_if_capable_max_pages(fuse_conn_info *conn_,
                             Config::Write  &cfg_)
   {
+    std::fstream f;
+    uint64_t max_pages_limit;
+
+    if(ghc::filesystem::exists(MAX_PAGES_LIMIT_FILEPATH))
+      {
+        if(cfg_->fuse_msg_size > MAX_FUSE_MSG_SIZE)
+          syslog_info("fuse_msg_size > %u: setting it to %u",
+                      MAX_FUSE_MSG_SIZE,
+                      MAX_FUSE_MSG_SIZE);
+        cfg_->fuse_msg_size = std::min((uint64_t)cfg_->fuse_msg_size,
+                                       (uint64_t)MAX_FUSE_MSG_SIZE);
+
+        f.open(MAX_PAGES_LIMIT_FILEPATH,f.in|f.out);
+        if(f.is_open())
+          {
+            f >> max_pages_limit;
+            syslog_info("%s currently set to %u",
+                        MAX_PAGES_LIMIT_FILEPATH,
+                        (uint64_t)max_pages_limit);
+            if(cfg_->fuse_msg_size > max_pages_limit)
+              {
+                f.seekp(0);
+                f << (uint64_t)cfg_->fuse_msg_size;
+                f.flush();
+                syslog_info("%s changed to %u",
+                            MAX_PAGES_LIMIT_FILEPATH,
+                            (uint64_t)cfg_->fuse_msg_size);
+              }
+            f.close();
+          }
+        else
+          {
+            if(cfg_->fuse_msg_size != FUSE_DEFAULT_MAX_MAX_PAGES)
+              syslog_info("unable to open %s",MAX_PAGES_LIMIT_FILEPATH);
+          }
+      }
+    else
+      {
+        if(cfg_->fuse_msg_size > FUSE_DEFAULT_MAX_MAX_PAGES)
+          syslog_info("fuse_msg_size request %u > %u: setting it to %u",
+                      (uint64_t)cfg_->fuse_msg_size,
+                      FUSE_DEFAULT_MAX_MAX_PAGES,
+                      FUSE_DEFAULT_MAX_MAX_PAGES);
+        cfg_->fuse_msg_size = std::min((uint64_t)cfg_->fuse_msg_size,
+                                       (uint64_t)FUSE_DEFAULT_MAX_MAX_PAGES);
+      }
+
     if(l::capable(conn_,FUSE_CAP_MAX_PAGES))
       {
         l::want(conn_,FUSE_CAP_MAX_PAGES);
         conn_->max_pages = cfg_->fuse_msg_size;
+        syslog_info("requesting max pages size of %u",
+                    (uint64_t)cfg_->fuse_msg_size);
       }
     else
       {