On Tue 03-09-19 08:10:15, Matthew Wilcox wrote: > On Tue, Sep 03, 2019 at 02:51:50PM +0200, Michal Hocko wrote: > > On Tue 03-09-19 05:22:08, Matthew Wilcox wrote: > > > On Tue, Sep 03, 2019 at 02:14:24PM +0200, Michal Hocko wrote: > > > > On Mon 02-09-19 03:23:41, William Kucharski wrote: > > > > > Add filemap_huge_fault() to attempt to satisfy page > > > > > faults on memory-mapped read-only text pages using THP when possible. > > > > > > > > This deserves much more description of how the thing is implemented and > > > > expected to work. For one thing it is not really clear to me why you > > > > need CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP at all. You need a support > > > > from the filesystem anyway. So who is going to enable/disable this > > > > config? > > > > > > There are definitely situations in which enabling this code will crash > > > the kernel. But we want to get filesystems to a point where they can > > > start working on their support for large pages. So our workaround is > > > to try to get the core pieces merged under a CONFIG_I_KNOW_WHAT_IM_DOING > > > flag and let people play with it. Then continue to work on the core > > > to eliminate those places that are broken. > > > > I am not sure I understand. Each fs has to opt in to the feature > > anyway. If it doesn't then there should be no risk of regression, right? > > I do not expect any fs would rush an implementation in while not being > > sure about the correctness. So how exactly does a config option help > > here. > > Filesystems won't see large pages unless they've opted into them. > But there's a huge amount of page-cache work that needs to get done > before this can be enabled by default. For example, truncate() won't > work properly. > > Rather than try to do all the page cache work upfront, then wait for the > filesystems to catch up, we want to get some basics merged. Since we've > been talking about this for so long without any movement in the kernel > towards actual support, this felt like a good way to go. > > We could, of course, develop the entire thing out of tree, but that's > likely to lead to pain and anguish. Then I would suggest mentioning all this in the changelog so that the overall intention is clear. It is also up to you fs developers to find a consensus on how to move forward. I have brought that up mostly because I really hate seeing new config options added due to shortage of confidence in the code. That really smells like working around standard code quality inclusion process. -- Michal Hocko SUSE Labs
