On Wed, Sep 13, 2017 at 02:02:29PM -0700, Andrew Morton wrote: > On Wed, 13 Sep 2017 10:40:19 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote: > > > Every zram users like low-end android device has used 0 page-cluster > > to disable swap readahead because it has no seek cost and works as > > synchronous IO operation so if we do readahead multiple pages, > > swap falut latency would be (4K * readahead window size). IOW, > > readahead is meaningful only if it doesn't bother faulted page's > > latency. > > > > However, this patch introduces additional knob /sys/kernel/mm/swap/ > > vma_ra_max_order as well as page-cluster. It means existing users > > has used disabled swap readahead doesn't work until they should be > > aware of new knob and modification of their script/code to disable > > vma_ra_max_order as well as page-cluster. > > > > I say it's a *regression* and wanted to fix it but Huang's opinion > > is that it's not a functional regression so userspace should be fixed > > by themselves. > > Please look into detail of discussion in > > http://lkml.kernel.org/r/%3C1505183833-4739-4-git-send-email-minchan@xxxxxxxxxx%3E > > hm, tricky problem. I do agree that linking the physical and virtual > readahead schemes in the proposed fashion is unfortunate. I also agree > that breaking existing setups (a bit) is also unfortunate. > > Would it help if, when page-cluster is written to zero, we do > > printk_once("physical readahead disabled, virtual readahead still > enabled. Disable virtual readhead via > /sys/kernel/mm/swap/vma_ra_max_order"). > > Or something like that. It's pretty lame, but it should help alert the > zram-readahead-disabling people to the issue? It was my last resort. If we cannot find other ways after all, yes, it would be a minimum we should do. But it still breaks users don't/can't read/modify alert and program. How about this? Can't we make vma-based readahead config option? With that, users who no interest on readahead don't enable vma-based readahead. In this case, page-cluster works as expected "disable readahead completely" so it doesn't break anything. People who want to use upcoming vma-based readahead can enable the feature and we can say such unfortunate things in config/document description somewhere so upcoming users will be aware of that unforunate two knobs. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
