On Tue, Sep 29, 2020 at 3:49 PM Marco Elver <elver@xxxxxxxxxx> wrote: > > On Tue, 29 Sep 2020 at 15:48, Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote: > > On Tue, Sep 29, 2020 at 3:11 PM Marco Elver <elver@xxxxxxxxxx> wrote: > > > > > > On Tue, Sep 29, 2020 at 02:42PM +0200, Andrey Konovalov wrote: > > > [...] > > > > > + */ > > > > > + index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1; > > > > > > > > Why do we subtract 1 here? We do have the metadata entry reserved for something? > > > > > > Above the declaration of __kfence_pool it says: > > > > > > * We allocate an even number of pages, as it simplifies calculations to map > > > * address to metadata indices; effectively, the very first page serves as an > > > * extended guard page, but otherwise has no special purpose. > > > > > > Hopefully that clarifies the `- 1` here. > > > > So there are two guard pages at the beginning and only then a page > > that holds an object? > > Yes, correct. OK, I see. This isn't directly clear from the comment though, at least for me :)
