On 2/4/19 10:19 AM, Matthew Wilcox wrote: > On Sun, Feb 03, 2019 at 09:21:33PM -0800, john.hubb...@gmail.com 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. >
Oh right, I'll just delete that last paragraph, then. Thanks for catching that. thanks, -- John Hubbard NVIDIA