On Thu, Sep 25, 2014 at 02:41:55PM +0200, Jan Kara wrote: > ->page_mkwrite() is used by filesystems to allocate blocks under a page > which is becoming writeably mmapped in some process' address space. This > allows a filesystem to return a page fault if there is not enough space > available, user exceeds quota or similar problem happens, rather than > silently discarding data later when writepage is called. > > However VFS fails to call ->page_mkwrite() in all the cases where > filesystems need it when blocksize < pagesize. For example when > blocksize = 1024, pagesize = 4096 the following is problematic: > ftruncate(fd, 0); > pwrite(fd, buf, 1024, 0); > map = mmap(NULL, 1024, PROT_WRITE, MAP_SHARED, fd, 0); > map[0] = 'a'; ----> page_mkwrite() for index 0 is called > ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */ > mremap(map, 1024, 10000, 0); > map[4095] = 'a'; ----> no page_mkwrite() called > > At the moment ->page_mkwrite() is called, filesystem can allocate only > one block for the page because i_size == 1024. Otherwise it would create > blocks beyond i_size which is generally undesirable. But later at > ->writepage() time, we also need to store data at offset 4095 but we > don't have block allocated for it. > > This patch introduces a helper function filesystems can use to have > ->page_mkwrite() called at all the necessary moments. > > Signed-off-by: Jan Kara <jack@xxxxxxx> Applied to the ext4 tree, thanks. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
