Re: [PATCH v1 10/15] mm/page-flags: reuse PG_slab as PG_anon_exclusive for PageAnon() pages

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 08, 2022 at 03:14:32PM +0100, David Hildenbrand wrote:
> The basic question we would like to have a reliable and efficient answer
> to is: is this anonymous page exclusive to a single process or might it
> be shared?

Is this supposed to be for PAGE_SIZE pages as well, or is it only used
on pages > PAGE_SIZE?

> In an ideal world, we'd have a spare pageflag. Unfortunately, pageflags
> don't grow on trees, so we have to get a little creative for the time
> being.

This feels a little _too_ creative to me.  There's now an implicit
requirement that SL[AOU]B doesn't use the bottom two bits of
->slab_cache, which is probably OK but would need to be documented.

I have plans to get rid of PageError and PagePrivate, but those are going
to be too late for you.  I don't think mappedtodisk has meaning for anon
pages, even if they're in the swapcache.  It would need PG_has_hwpoisoned
to shift to another bit ... but almost any bit will do for has_hwpoisoned.
Or have I overlooked something?

> @@ -920,6 +976,70 @@ extern bool is_free_buddy_page(struct page *page);
>  
>  __PAGEFLAG(Isolated, isolated, PF_ANY);
>  
> +static __always_inline bool folio_test_slab(struct folio *folio)
> +{
> +	return !folio_test_anon(folio) &&
> +	       test_bit(PG_slab, folio_flags(folio, FOLIO_PF_NO_TAIL));
> +}
> +
> +static __always_inline int PageSlab(struct page *page)
> +{
> +	return !PageAnon(page) &&
> +		test_bit(PG_slab, &PF_NO_TAIL(page, 0)->flags);
> +}

In case we do end up using this, this would be better implemented as

static __always_inline int PageSlab(struct page *page)
{
	return folio_test_slab(page_folio(page));
}

since PageAnon already has a page_folio() call embedded in it.

> +static __always_inline void __SetPageSlab(struct page *page)
> +{
> +	VM_BUG_ON_PGFLAGS(PageAnon(page), page);
> +	__set_bit(PG_slab, &PF_NO_TAIL(page, 1)->flags);
> +}

There's only one caller of __SetPageSlab() left, in kfence.  And that
code looks ... weird.

        for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
                if (!i || (i % 2))
                        continue;

                /* Verify we do not have a compound head page. */
                if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
                        goto err;

                __SetPageSlab(&pages[i]);

I think the author probably intended WARN_ON(PageCompound(page)) because
they're actually verifying that it's not a tail page, rather than head
page.

> +static __always_inline void __ClearPageSlab(struct page *page)
> +{
> +	VM_BUG_ON_PGFLAGS(PageAnon(page), page);
> +	__clear_bit(PG_slab, &PF_NO_TAIL(page, 1)->flags);
> +}

There are no remaining callers of __ClearPageSlab().  yay.