On Thu, 27 Jan 2022, Hugh Dickins wrote: > On Thu, 27 Jan 2022, NeilBrown wrote: > > On Mon, 24 Jan 2022, Christoph Hellwig wrote: > > > On Mon, Jan 24, 2022 at 02:48:32PM +1100, NeilBrown wrote: > > > > Code that does swap read-ahead uses blk_start_plug() and > > > > blk_finish_plug() to allow lower levels to combine multiple read-ahead > > > > pages into a single request, but calls blk_finish_plug() *before* > > > > submitting the original (non-ahead) read request. > > > > This missed an opportunity to combine read requests. > > No, you're misunderstanding there. All the necessary reads are issued > within the loop, between the plug and unplug: it does not skip over > the target page in the loop, but issues its read along with the rest. > > But it has not kept any of those pages locked, nor even kept any > refcounts raised: so at the end has to look up the target page again > with the final read_swap_cache_async() (which also copes with the > highly unlikely case that the page got swapped out again meanwhile). > .... > > I don't suppose your patch does any actual harm (beyond propagating a > misunderstanding), but it's certainly not a fix, and I think should > simply be dropped from the series. Thanks - I had missed that. The code is correct, but looks wrong (to me). I've dropped the patch, but added a comment when I add "swap_read_unplug()" to explain while plugging isn't needed for that final read_swap_cache_async(). > > (But please don't expect any comment from me on the rest: > SWP_FS_OPS has always been beyond my understanding.) :-) Thanks, NeilBrown
