On 7:01 07/07, Darrick J. Wong wrote: > On Tue, Jul 07, 2020 at 08:49:52AM -0500, Goldwyn Rodrigues wrote: > > On 13:57 07/07, Matthew Wilcox wrote: > > > On Tue, Jul 07, 2020 at 07:43:46AM -0500, Goldwyn Rodrigues wrote: > > > > On 9:53 01/07, Christoph Hellwig wrote: > > > > > On Mon, Jun 29, 2020 at 02:23:49PM -0500, Goldwyn Rodrigues wrote: > > > > > > From: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx> > > > > > > > > > > > > For direct I/O, add the flag IOMAP_DIO_RWF_NO_STALE_PAGECACHE to indicate > > > > > > that if the page invalidation fails, return back control to the > > > > > > filesystem so it may fallback to buffered mode. > > > > > > > > > > > > Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > > > > > > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx> > > > > > > > > > > I'd like to start a discussion of this shouldn't really be the > > > > > default behavior. If we have page cache that can't be invalidated it > > > > > actually makes a whole lot of sense to not do direct I/O, avoid the > > > > > warnings, etc. > > > > > > > > > > Adding all the relevant lists. > > > > > > > > Since no one responded so far, let me see if I can stir the cauldron :) > > > > > > > > What error should be returned in case of such an error? I think the > > > > > > Christoph's message is ambiguous. I don't know if he means "fail the > > > I/O with an error" or "satisfy the I/O through the page cache". I'm > > > strongly in favour of the latter. Indeed, I'm in favour of not invalidating > > > the page cache at all for direct I/O. For reads, I think the page cache > > > should be used to satisfy any portion of the read which is currently > > > > That indeed would make reads faster. How about if the pages are dirty > > during DIO reads? > > Should a direct I/O read be responsible for making sure that the dirty > > pages are written back. Technically direct I/O reads is that we are > > reading from the device. > > The filemap_write_and_wait_range should persist that data, right? Right. filemap_write_and_wait_range() would not make sense for writes though. > > > > cached. For writes, I think we should write into the page cache pages > > > which currently exist, and then force those pages to be written back, > > > but left in cache. > > > > Yes, that makes sense. > > If this is implemented, what would be the difference between O_DIRECT > > and O_DSYNC, if any? > > Presumably a direct write would proceed as it does today if there's no > pagecache at all? > Yes, correct. Just that it would leave pages in the cache instead of invalidating it after DIO write is complete. -- Goldwyn
