pagecache: report page-writeback errors from sync() by return value

core/pagecache.cc

@@ -957,7 +957,15 @@ bool release(vfs_file* fp, void *addr, off_t offset, mmu::hw_ptep<0> ptep)
     return addr != zero_page;
 }
 
-void sync(vfs_file* fp, off_t start, off_t end)
+// Flush dirty MAP_SHARED pages for [start,end) back to the filesystem.
+// Returns 0 on success or the writeback errno.  Historically this threw a C++
+// error on writeback failure, but it is reached from munmap() (via
+// mmu::file_vma::sync) as well as from an explicit fsync path, and throwing on
+// the munmap path is both unnecessary (the caller converts the result to a
+// return value) and unsafe: an exception raised there can abort when the C++
+// unwinder cannot run in that context.  Report the error by return value
+// instead and let each caller decide how to surface it.
+int sync(vfs_file* fp, off_t start, off_t end)
 {
     struct stat st;
     fp->stat(&st);
@@ -989,20 +997,23 @@ void sync(vfs_file* fp, off_t start, off_t end)
     }
 
     if (to_flush.empty())
-        return;
+        return 0;
 
     mmu::flush_tlb_all();
 
     /* Phase 3: write each page back to the filesystem. */
+    int ret = 0;
     for (auto cp : to_flush) {
         auto err = cp->writeback();
         if (err) {
             // Re-mark dirty: phase 2 cleared the flag, but the data never
             // reached the filesystem, so it must not be treated as clean.
             cp->mark_dirty();
-            throw make_error(err);
+            if (!ret)
+                ret = err;
         }
     }
+    return ret;
 }
 
 /*

fs/vfs/vfs_fops.cc

@@ -177,7 +177,10 @@ bool vfs_file::put_page(void *addr, uintptr_t off, mmu::hw_ptep<0> ptep)
 
 void vfs_file::sync(off_t start, off_t end)
 {
-    pagecache::sync(this, start, end);
+    // pagecache::sync() now reports writeback errors by return value rather
+    // than throwing; the munmap/msync path (mmu::file_vma::sync) surfaces any
+    // error via its own sys_fsync() tail, so no caller depends on a throw here.
+    (void) pagecache::sync(this, start, end);
 }
 
 // Locking: VOP_CACHE will call into the filesystem, and that can trigger an

include/osv/file.h

@@ -168,7 +168,7 @@ struct file {
 	virtual bool map_page(uintptr_t offset, mmu::hw_ptep<1> ptep, mmu::pt_element<1> pte, bool write, bool shared) { throw make_error(ENOSYS); }
 	virtual bool put_page(void *addr, uintptr_t offset, mmu::hw_ptep<0> ptep) { throw make_error(ENOSYS); }
 	virtual bool put_page(void *addr, uintptr_t offset, mmu::hw_ptep<1> ptep) { throw make_error(ENOSYS); }
-	virtual void sync(off_t start, off_t end) { throw make_error(ENOSYS); }
+	virtual void sync(off_t start, off_t end) { /* default: nothing to sync */ }
 
 	int		f_flags;	/* open flags */
 	int		f_count;	/* reference count, see below */

include/osv/pagecache.hh

@@ -39,7 +39,7 @@ bool release(vfs_file* fp, void *addr, off_t offset, mmu::hw_ptep<0> ptep);
  * sync() — flush all dirty pages in [start, end) for the file described by fp.
  * Throws on I/O error.  Used by VOP_FSYNC implementations.
  */
-void sync(vfs_file* fp, off_t start, off_t end);
+int sync(vfs_file* fp, off_t start, off_t end);
 
 /*
  * writeback_inode() — flush dirty write-cache pages for a specific (dev, ino)