Re: [PATCH 1/3] mm/gup: track FOLL_PIN pages

Jan Kara
2020-01-27T11:24:02Z

On Fri 24-01-20 18:11:13, John Hubbard wrote:
> +static __maybe_unused struct page *try_grab_compound_head(struct page *page,
> + int refs,
> + unsigned int flags)
> +{
> + if (flags & FOLL_GET)
> + return try_get_compound_head(page, refs);
> + else if (flags & FOLL_PIN) {
> + int orig_refs = refs;
> +
> + /*
> + * When pinning a compound page of order > 1 (which is what
> + * hpage_pincount_available() checks for), use an exact count to
> + * track it, via hpage_pincount_inc/_dec().
> + *
> + * However, be sure to *also* increment the normal page refcount
> + * field at least once, so that the page really is pinned.
> + */
> + if (!hpage_pincount_available(page))
> + refs *= GUP_PIN_COUNTING_BIAS;
> +
> + page = try_get_compound_head(page, refs);
> + if (!page)
> + return NULL;
> +
> + if (hpage_pincount_available(page))
> + hpage_pincount_inc(page);

Umm, adding just 1 to pincount looks dangerous to me as
try_grab_compound_head() would not compose - you could not release
references acquired by try_grab_compound_head() with refs==2 by two calls
to put_compound_head() with refs==1. So I'd rather have here:
hpage_pincount_add(page, refs) and similarly in put_compound_head().
Otherwise the patch looks good to me from a quick look.

Honza

--
Jan Kara <jack@suse.com>
SUSE Labs, CR