On Tue 17-05-11 11:11:08, Christoph Hellwig wrote: > On Wed, May 11, 2011 at 12:29:34AM +0200, Jan Kara wrote: > > We should not allow file modification via mmap while the filesystem is > > frozen. So block in ext4_page_mkwrite() while the filesystem is frozen. > > > > We have to check for frozen filesystem with the page marked dirty and under > > page lock with which we then return from ext4_page_mkwrite(). Only that way we > > cannot race with writeback done by freezing code - either we mark the page > > dirty after the writeback has started, see freezing in progress and block, or > > writeback will wait for our page lock which is released only when the fault is > > done and then writeback will writeout and writeprotect the page again. > > This really should be done in (__)block_page_mkwrite. I'd also return > VM_FAULT_RETRY instead of retrying inside the block_mkwrite handler > in case you hit the race. Well, we can do the non-blocking check at the end of __block_page_mkwrite() and return some error value (EAGAIN translating to VM_FAULT_RETRY would look logical, I just have to think off better error value for VM_FAULT_NOPAGE). But vfs_check_frozen() cannot be in __block_page_mkwrite() since ext4 needs to call that with a transaction started so that would create a deadlock and we need to call vfs_check_frozen() somewhere so that we don't busyloop. I can call vfs_check_frozen() inside block_page_mkwrite() but it would be a bit surprising difference from __block_page_mkwrite() to me. Not sure what the cleanest solution would be here... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
