On Mon, Aug 14, 2023 at 08:08:57PM +0100, Matthew Wilcox wrote: > On Mon, Aug 14, 2023 at 02:44:09PM -0400, Peter Xu wrote: > > +/* > > + * This macro defines the maximum tail pages (of a folio) that can have the > > + * page->mapping field reused (offset 12 for 32bits, or 24 for 64bits). > > No, don't say how many bytes into the structure something is. It'll > only get out of date. If somebody needs to know, use pahole. OK. > > > + * 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. One should at least also need to change __split_huge_page_tail() on the BUG_ON() with the hard-coded "tail > 2"? I wanted to link all these pieces together, and the use case is when anyone would like to e.g. reuse tail page 3 of a folio, hence put the macro here. -- Peter Xu
