Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
> PowerPC has special handling of hugetlbfs pages. Well, that's what
> the config option says, but actually it handles THP as well. If
> the config option is enabled.
>
> #ifdef CONFIG_HUGETLB_PAGE
> if (PageCompound(page)) {
> flush_dcache_icache_hugepage(page);
> return;
> }
> #endif
I do have a change posted sometime back to avoid that confusion.
http://patchwork.ozlabs.org/project/linuxppc-dev/patch/20200320103256.229365-1-aneesh.kumar@xxxxxxxxxxxxx/
But IIUC we use the head page flags (PG_arch_1) to track whether we need
the flush or not.
>
> By the way, THPs can be mapped askew -- that is, at an offset which
> means you can't use a PMD to map a PMD sized page.
>
> Anyway, we don't really have consensus between the various architectures
> on how to handle either THPs or hugetlb pages. It's not contemplated
> in Documentation/core-api/cachetlb.rst so there's no real surprise
> we've diverged.
>
> What would you _like_ to see? Would you rather flush_dcache_page()
> were called once for each subpage, or would you rather maintain
> the page-needs-flushing state once per compound page? We could also
> introduce flush_dcache_thp() if some architectures would prefer it one
> way and one the other, although that brings into question what to do
> for hugetlbfs pages.
>
> It might not be a bad idea to centralise the handling of all this stuff
> somewhere. Sounds like the kind of thing Arnd would like to do ;-) I'll
> settle for getting enough clear feedback about what the various arch
> maintainers want that I can write a documentation update for cachetlb.rst.
