amd64/arm64 pmap: consistently clear PGA_WRITEABLE on fictitious, managed pages

[alc]

Summary

While optimizing icache management on arm64, I found that we don't consistently clear PGA_WRITEABLE on fictitious, managed pages. Some functions do, e.g., pmap_remove_all(), but several do not. At worst, this is just a pessimization, but there is no good reason to be inconsistent Clear PGA_WRITEABLE in those that previously did not.

[kib]

sys/amd64/amd64/pmap.c

5321

Is it worth making a static function like this

static void
pmap_might_be_clear_writeable(vm_page_t m)
{
    if (!TAILQ_EMPTY(&m->md.pv_list))
        return;
    if ((m->flags & PG_FICTITIOUS) == 0 &&
        !TAILQ_EMPTY(&pa_to_pvh(tpte & PG_FRAME)->pv_list)
           return;
      vm_page_aflag_clear(m, PGA_WRITEABLE);
}

[alc]

Introduce pmap_page_is_mapped_locked().

[markj]

sys/amd64/amd64/pmap.c

7279

Some sites check whether PGA_WRITEABLE is set before checking whether or not to clear it. Others don't, but I can't see a good reason for the inconsistency.

[alc]

sys/amd64/amd64/pmap.c

7279

This one is the easiest to explain. It is most likely replacing a mapping to a COW page, and the additional check avoids what would be a pointless atomic. The other is pmap_remove_pages(), where we are also destroying a lot of R/O and COW mappings. In the other cases, I think it is less clear what type of mapping it is.

[alc]

sys/amd64/amd64/pmap.c

7279

Would you like me to add /* The old page is likely COW. */ here?

[kib]

sys/amd64/amd64/pmap.c

7279

'... so check for writeable first' or like?