Hi, below is a patch series that is my new approach to solve problems with page_mkwrite() when blocksize < pagesize. To refresh memory the main issue is as follows: We'd like to use page_mkwrite() 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. On filesystems where blocksize < pagesize the situation is complicated though. Think for example that blocksize = 1024, pagesize = 4096 and a process does: ftruncate(fd, 0); pwrite(fd, buf, 1024, 0); map = mmap(NULL, 4096, 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) */ fsync(fd); ----> writepage() for index 0 is 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 would like to have blocks allocated for the whole page (and in principle we have to allocate them because user could have filled the page with data after the second ftruncate()). This series is an attempt to fix the above issue. The idea is that we do i_size update after an extending write or truncate not under the page lock of the page where the i_size ends up but under the page lock of the page where i_size was originally. This also allows us to solve a posix compliance issue where we could have exposed data written via mmap beyond i_size. I see two disputable things with this approach: 1) set_page_dirty_buffers() and create_empty_buffers() now checks i_size. That's a bit ugly although not marking buffers dirty beyond i_size makes a lot of sence to me. 2) to fix the problem with non-zeros written via mmap beyond EOF and then being exposed by truncate, I've added zeroing to a function doing all the work when extending i_size (which is essentially the only place where we can reliably do the work and avoid races with mmap). That's a good fit but basically all filesystems now have to extend i_size with this function which I don't find that pleasing. Anybody has an idea how to avoid that conversion of every filesystem or make it less painful? Thanks for comments in advance. Honza -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
