Why does juicefs implement its own openfiles logic to determine whether to set the keep-cache attribute of a file?

[daimashusheng]

According to linux kernel fuse related code, in CAP_AUTO_INVAL_DATA mode, if the size or modification time of the file has changed when it is read again, the kernel will automatically clear the previous page cache, so as to read the real content of the file. Therefore, if juicefs directly set keep-cache to true, we can let the kernel to automatically clear the expired page cache, I do not know why juicefs also implement its own openfiles logic to determine whether to set the keep-cache attribute of a file?

This is source code for linux kernel fuse to determine if page cache is valid.

		bool inval = false;

		if (oldsize != attr->size) {
			truncate_pagecache(inode, attr->size);
			if (!fc->explicit_inval_data)
				inval = true;
		} else if (fc->auto_inval_data) {
			struct timespec64 new_mtime = {
				.tv_sec = attr->mtime,
				.tv_nsec = attr->mtimensec,
			};

			/*
			 * Auto inval mode also checks and invalidates if mtime
			 * has changed.
			 */
			if (!timespec64_equal(&old_mtime, &new_mtime))
				inval = true;
		}

		if (inval)
			invalidate_inode_pages2(inode->i_mapping);

or https://github.com/torvalds/linux/blob/master/fs/fuse/inode.c#L340