Matthew Wilcox [off-list ref] 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@linux.ibm.com/

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.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel