On Thu, Feb 08, 2007 at 01:44:15AM +1100, David Chinner wrote: > On Wed, Feb 07, 2007 at 01:00:28PM +0000, Hugh Dickins wrote: > > On Wed, 7 Feb 2007, David Chinner wrote: > > > > > On Christoph's suggestion, take the guts of the proposed > > > xfs_vm_page_mkwrite function and implement it as a generic > > > core function as it used no specific XFS code at all. > > > > > > This allows any filesystem to easily hook the ->page_mkwrite() > > > VM callout to allow them to set up pages dirtied by mmap > > > writes correctly for later writeout. > > > > > > Signed-Off-By: Dave Chinner <dgc@xxxxxxx> > > > > I'm worried about concurrent truncation. Isn't it the case that > > i_mutex is held when prepare_write and commit_write are normally > > called? But not here when page_mkwrite is called. > > I'm not holding i_mutex. I assumed that it was probably safe to do > because we are likely to be reading the page off disk just before we > call mkwrite and that has to be synchronised with truncate in some > manner.... In general, commit_write is allowed to update i_size, and prepare/commit are called with i_mutex. block_prepare_write and block_commit_write both look safe to me for calling with only the page lock held. It more or less translates to: call get_block in a sane fashion and zero out the parts of the page past eof. But, if someone copies the code and puts their own fancy prepare/commit_write in there, they will get in trouble in a hurry... > > So, do I need to grab the i_mutex here? Is that safe to do that in > the middle of a page fault? If we do race with a truncate and the > page is now beyond EOF, what am I supposed to return? Should it check to make sure the page is still in the address space after locking it? -chris - 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
