.

wcfs/notes.txt

@@ -20,18 +20,22 @@ we have to disable FUSE_AUTO_INVAL_DATA to tell the kernel we are fully
 responsible for invalidating pagecache. If we don't, the kernel will be
 clearing whole cache of head/file on e.g. its mtime change.
 
-XXX FUSE_AUTO_INVAL_DATA does not fully prevent kernel from automatically
+Note: FUSE_AUTO_INVAL_DATA does not fully prevent kernel from automatically
 invalidating pagecache - e.g. it will invalidate whole cache on file size changes:
 
 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/fuse/inode.c?id=e0bc833d10#n233
 
-(XXX no) we can currently workaround it with using writeback mode (see !is_wb in the
-link above), but better we have proper FUSE flag for filesystem server to
-tell the kernel it is fully responsible for invalidating pagecache.
+It was hoped that we could workaround it with using writeback mode (see !is_wb
+in the link above), but it turned out that in writeback mode the kernel indeed
+does not invalidate data cache on file size change, but neither it allows the
+filesystem to set the size due to external event (see https://git.kernel.org/linus/8373200b12
+"fuse: Trust kernel i_size only"). This prevents us to use writeback workaround
+as we cannot even update the file from being empty to having some data.
 
-XXX ^^^ we cannot workaround with writeback mode - in writeback mode the kernel
-does not trust us for size that we report via getattr, and thus we cannot even
-update the file from being 0 size to having some data.
+-> we did the patch for FUSE to have proper flag for filesystem server to tell
+the kernel it is fully responsible for invalidating pagecache:
+
+https://lwn.net/ml/linux-fsdevel/20190315212556.9315-1-kirr%40nexedi.com/
 
 
 Invalidations to wcfs clients are delayed until block access