On Thu, Jul 03, 2025 at 06:52:44AM -0700, Christoph Hellwig wrote: > On Tue, Jul 01, 2025 at 10:48:47PM +0800, alexjlzheng@xxxxxxxxx wrote: > > From: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx> > > > > In the buffer write path, iomap_set_range_uptodate() is called every > > time iomap_end_write() is called. But if folio_test_uptodate() holds, we > > know that all blocks in this folio are already in the uptodate state, so > > there is no need to go deep into the critical section of state_lock to > > execute bitmap_set(). > > > > Although state_lock may not have significant lock contention due to > > folio lock, this patch at least reduces the number of instructions. > > That means the uptodate bitmap is stale in that case. That would > only matter if we could clear the folio uptodate bit and still > expect the page content to survive. Which sounds dubious and I could > not find anything relevant grepping the tree, but I'm adding the > linux-mm list just in case. Once a folio is uptodate, there is no route back to !uptodate without going through the removal of the folio from the page cache. The read() path relies on this for example; once it has a refcount on the folio, and has checked the uptodate bit, it will copy the contents to userspace.
