On Fri, Aug 18, 2023 at 11:14:28PM +0100, Matthew Wilcox wrote: > On Tue, Aug 15, 2023 at 05:06:59PM -0400, Peter Xu wrote: > > I split this out from another rfc series. Removed RFC tag because it > > wasn't for this patch but for the documentation updates. I'll post the rfc > > part alone. Comments welcomed, thanks. > > I still hate it, as I explained here: I still prefer it be merged. > > https://lore.kernel.org/linux-mm/ZNp7yUgUrIpILnXu@xxxxxxxxxxxxxxxxxxxx/ https://lore.kernel.org/linux-mm/ZNqFv0AwkfDKExiw@x1n/#t Firstly, I've answered and you didn't follow that up. This is not identical to that version, it added one more change to remove the other hard coded "2" > > > > + * When the tail page's mapping field reused, it'll be exempted from > > > + * ->mapping poisoning and checks. Also see the macro TAIL_MAPPING. > > > + */ > > > +#define TAIL_MAPPING_REUSED_MAX (2) > > > > More importantly, I think this is over-parametrisation. If you start to > > use extra fields in struct folio, just change the code in page_alloc.c > > directly. Change the hard-coded "2"s in different functions? Can you kindly explain why can't we just have a macro to help? Setting tail mapping for tail 1/2 is even wrong, which part of this patch fixes: @@ -428,7 +428,8 @@ static inline void prep_compound_tail(struct page *head, int tail_idx) { struct page *p = head + tail_idx; - p->mapping = TAIL_MAPPING; + if (tail_idx > TAIL_MAPPING_REUSED_MAX) + p->mapping = TAIL_MAPPING; set_compound_head(p, head); set_page_private(p, 0); } Do you mean you prefer me to add one more hard-coded "2" and "just boost it when it's needed"? Thanks, -- Peter Xu
