In a sense this question is premature, because we don't have any code in place to handle folios which are any size but PMD_SIZE or PAGE_SIZE, but let's pretend that code already exists and is just waiting for us to answer this policy question. I'd like to reject three ideas up front: 1. a CONFIG option, 2. a boot option and 3. a sysfs tunable. It is foolish to expect the distro packager or the sysadmin to be able to make such a decision. The correct decision will depend upon the instantaneous workload of the entire machine and we'll want different answers for different VMAs. I'm open to applications having some kind of madvise() call they can use to specify hints, but I would prefer to handle memory efficiently for applications which do not. For pagecache memory, we use the per-fd readahead code; if readahead has been successful in the past we bump up the folio size until it reaches its maximum. There is no equivalent for anonymous memory. I'm working my way towards a solution that looks a little like this: A. We modify khugepaged to quadruple the folio size each time it scans. At the moment, it always attempts to promote straight from order 0 to PMD size. Instead, if it finds four adjacent order-0 folios, it will allocate an order-2 folio to replace them. Next time it scans, it finds four order-2 folios and replaces them with a single order-4 folio. And so on, up to PMD order. B. A further modification is that it will require three of the four folios being combined to be on the active list. If two (or more) of the four folios are inactive, we should leave them alone; either they will remain inactive and eventually be evicted, or they will be activated and eligible for merging in a future pass of khugepaged. C. We add a new wrinkle to the LRU handling code. When our scan of the active list examines a folio, we look to see how many of the PTEs mapping the folio have been accessed. If it is fewer than half, and those half are all in either the first or last half of the folio, we split it. The active half stays on the active list and the inactive half is moved to the inactive list. I feel that these three changes should allow us to iterate towards a solution for any given VMA that is close to optimal, and adapts to a changing workload with no intervention from a sysadmin, or even hint from a program. There are three different circumstances where we currently allocate anonymous memory. The first is for mmap(MAP_ANONYMOUS), the second is COW on a file-backed MAP_PRIVATE and the third is COW of a post-fork anonymous mapping. For the first option, the only hint we have is the size of the VMA. I'm tempted to suggest our initial guess at the right size folio to allocate should be scaled to that, although I don't have a clear idea about what the scale factor should be. For the second case, I want to strongly suggest that the size of the folio allocated by the page cache should be of no concern. It is largely irrelevant to the application's usage pattern what size the page cache has chosen to cache the file. I might start out very conservatively here with an order-0 allocation. For the third case, in contrast, the parent had already established an appropriate size folio to use for this VMA before calling fork(). Whether it is the parent or the child causing the COW, it should probably inherit that choice and we should default to the same size folio that was already found. I don't stay current with the research literature, so if someone wants to point me to a well-studied algorithm and let me know that I can stop thinking about this, that'd be great. And if anyone wants to start working on implementing this, that'd also be great. P.S. I didn't want to interrupt the flow of the above description to note that allocation of any high-order folio can and will fail, so there will definitely be fallback points to order-0 folios, which will be no different from today. Except that maybe we'll be able to iterate towards the correct folio size in the new khugepaged. P.P.S. I still consider myself a bit of a novice in the handling of anonymous memory, so don't be shy to let me know what I got wrong.
