Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > On Fri, Jul 07, 2023 at 01:29:02PM +0200, David Hildenbrand wrote: >> On 07.07.23 11:52, Ryan Roberts wrote: >> > On 07/07/2023 09:01, Huang, Ying wrote: >> > > Although we can use smaller page order for FLEXIBLE_THP, it's hard to >> > > avoid internal fragmentation completely. So, I think that finally we >> > > will need to provide a mechanism for the users to opt out, e.g., >> > > something like "always madvise never" via >> > > /sys/kernel/mm/transparent_hugepage/enabled. I'm not sure whether it's >> > > a good idea to reuse the existing interface of THP. >> > >> > I wouldn't want to tie this to the existing interface, simply because that >> > implies that we would want to follow the "always" and "madvise" advice too; That >> > means that on a thp=madvise system (which is certainly the case for android and >> > other client systems) we would have to disable large anon folios for VMAs that >> > haven't explicitly opted in. That breaks the intention that this should be an >> > invisible performance boost. I think it's important to set the policy for use of >> >> It will never ever be a completely invisible performance boost, just like >> ordinary THP. >> >> Using the exact same existing toggle is the right thing to do. If someone >> specify "never" or "madvise", then do exactly that. >> >> It might make sense to have more modes or additional toggles, but >> "madvise=never" means no memory waste. > > I hate the existing mechanisms. They are an abdication of our > responsibility, and an attempt to blame the user (be it the sysadmin > or the programmer) of our code for using it wrongly. We should not > replicate this mistake. > > Our code should be auto-tuning. I posted a long, detailed outline here: > https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@xxxxxxxxxxxxxxxxxxxx/ Yes. Auto-tuning should be more preferable than any configuration mechanisms. Something like THP shrinker could be another way of auto-tuning. https://lore.kernel.org/linux-mm/cover.1667454613.git.alexlzhu@xxxxxx/ That is, allocating the large folios on page fault, then try to detect internal fragmentation. >> I remember I raised it already in the past, but you *absolutely* have to >> respect the MADV_NOHUGEPAGE flag. There is user space out there (for >> example, userfaultfd) that doesn't want the kernel to populate any >> additional page tables. So if you have to respect that already, then also >> respect MADV_HUGEPAGE, simple. > > Possibly having uffd enabled on a VMA should disable using large folios, > I can get behind that. But the notion that userspace knows what it's > doing ... hahaha. Just ignore the madvise flags. Userspace doesn't > know what it's doing. Best Regards, Huang, Ying
