On 5/23/24 3:11 PM, Matthew Wilcox wrote: > On Thu, May 23, 2024 at 01:08:02PM -0700, Andrew Morton wrote: >> On Mon, 13 May 2024 21:23:39 +0800 Liu Wei <liuwei09@xxxxxxxx> wrote: >> >>> After commit (6be96d3ad3 fs: return if direct I/O will trigger writeback), >>> when we issuing AIO with direct I/O and IOCB_NOWAIT on a block device, the >>> process context will not be blocked. >>> >>> However, if the device already has page cache in memory, EAGAIN will be >>> returned. And even when trying to reissue the AIO with direct I/O and >>> IOCB_NOWAIT again, we consistently receive EAGAIN. >>> >>> Maybe a better way to deal with it: filemap_fdatawrite_range dirty pages >>> with WB_SYNC_NONE flag, and invalidate_mapping_pages unmapped pages at >>> the same time. >> >> Can't userspace do this? If EAGAIN, sync the fd and retry the IO? > > I don't think that it can, because the pages will still be there, even > if now clean? I think the idea was to punt to a worker thread which > could sleep and retry without NOWAIT. But let's see what someone > involved in this patch has to say about the intent. Right, the idea is that if you get -EAGAIN, a non-blocking attempt wasn't possible. You'd need to retry from somewhere where you CAN block. Any issuer very much can do that, whether it's in-kernel or not. It'd be somewhat fragile to make assumptions on what can cause the -EAGAIN and try to rectify them, and then try again with IOCB_NOWAIT. -- Jens Axboe
