On Mon, 2021-04-05 at 20:31 +0100, Matthew Wilcox wrote: > On Mon, Apr 05, 2021 at 03:14:29PM -0400, Jeff Layton wrote: > > On Wed, 2021-03-31 at 19:47 +0100, Matthew Wilcox (Oracle) wrote: > > > Managing memory in 4KiB pages is a serious overhead. Many benchmarks > > > exist which show the benefits of a larger "page size". As an example, > > > an earlier iteration of this idea which used compound pages got a 7% > > > performance boost when compiling the kernel using kernbench without any > > > particular tuning. > > > > > > Using compound pages or THPs exposes a serious weakness in our type > > > system. Functions are often unprepared for compound pages to be passed > > > to them, and may only act on PAGE_SIZE chunks. Even functions which are > > > aware of compound pages may expect a head page, and do the wrong thing > > > if passed a tail page. > > > > > > There have been efforts to label function parameters as 'head' instead > > > of 'page' to indicate that the function expects a head page, but this > > > leaves us with runtime assertions instead of using the compiler to prove > > > that nobody has mistakenly passed a tail page. Calling a struct page > > > 'head' is also inaccurate as they will work perfectly well on base pages. > > > The term 'nottail' has not proven popular. > > > > > > We also waste a lot of instructions ensuring that we're not looking at > > > a tail page. Almost every call to PageFoo() contains one or more hidden > > > calls to compound_head(). This also happens for get_page(), put_page() > > > and many more functions. There does not appear to be a way to tell gcc > > > that it can cache the result of compound_head(), nor is there a way to > > > tell it that compound_head() is idempotent. > > > > > > This series introduces the 'struct folio' as a replacement for > > > head-or-base pages. This initial set reduces the kernel size by > > > approximately 5kB by removing conversions from tail pages to head pages. > > > The real purpose of this series is adding infrastructure to enable > > > further use of the folio. > > > > > > The medium-term goal is to convert all filesystems and some device > > > drivers to work in terms of folios. This series contains a lot of > > > explicit conversions, but it's important to realise it's removing a lot > > > of implicit conversions in some relatively hot paths. There will be very > > > few conversions from folios when this work is completed; filesystems, > > > the page cache, the LRU and so on will generally only deal with folios. > > > > I too am a little concerned about the amount of churn this is likely to > > cause, but this does seem like a fairly promising way forward for > > actually using THPs in the pagecache. The set is fairly straightforward. > > > > That said, there are few callers of these new functions in here. Is this > > set enough to allow converting some subsystem to use folios? It might be > > good to do that if possible, so we can get an idea of how much work > > we're in for. > > It isn't enough to start converting much. There needs to be a second set > of patches which add all the infrastructure for converting a filesystem. > Then we can start working on the filesystems. I have a start at that > here: > > https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/folio > > I don't know if it's exactly how I'll arrange it for submission. It might > be better to convert all the filesystem implementations of readpage > to work on a folio, and then the big bang conversion of ->readpage to > ->read_folio will look much more mechanical. > > But if I can't convince people that a folio approach is what we need, > then I should stop working on it, and go back to fixing the endless > stream of bugs that the thp-based approach surfaces. Fair enough. I generally prefer to see some callers added at the same time as new functions, but I understand that the scale of this patchset makes that difficult. You can add this to the whole series. I don't see any major show-stoppers here: Acked-by: Jeff Layton <jlayton@xxxxxxxxxx>
