Re: [PATCH 4/6] mm/gup: track gup-pinned pages

[Matthew Wilcox]

On Sun, Feb 03, 2019 at 09:21:33PM -0800, john.hubbard@xxxxxxxxx wrote:
> +/*
> + * GUP_PIN_COUNTING_BIAS, and the associated functions that use it, overload
> + * the page's refcount so that two separate items are tracked: the original page
> + * reference count, and also a new count of how many get_user_pages() calls were
> + * made against the page. ("gup-pinned" is another term for the latter).
> + *
> + * With this scheme, get_user_pages() becomes special: such pages are marked
> + * as distinct from normal pages. As such, the new put_user_page() call (and
> + * its variants) must be used in order to release gup-pinned pages.
> + *
> + * Choice of value:
> + *
> + * By making GUP_PIN_COUNTING_BIAS a power of two, debugging of page reference
> + * counts with respect to get_user_pages() and put_user_page() becomes simpler,
> + * due to the fact that adding an even power of two to the page refcount has
> + * the effect of using only the upper N bits, for the code that counts up using
> + * the bias value. This means that the lower bits are left for the exclusive
> + * use of the original code that increments and decrements by one (or at least,
> + * by much smaller values than the bias value).
> + *
> + * Of course, once the lower bits overflow into the upper bits (and this is
> + * OK, because subtraction recovers the original values), then visual inspection
> + * no longer suffices to directly view the separate counts. However, for normal
> + * applications that don't have huge page reference counts, this won't be an
> + * issue.
> + *
> + * This has to work on 32-bit as well as 64-bit systems. In the more constrained
> + * 32-bit systems, the 10 bit value of the bias value leaves 22 bits for the
> + * upper bits. Therefore, only about 4M calls to get_user_page() may occur for
> + * a page.

The refcount is 32-bit on both 64 and 32 bit systems.  This limit
exists on both sizes of system.