On 27 November 2015 at 11:02, Will Deacon <will.deacon@xxxxxxx> wrote: > On Fri, Nov 27, 2015 at 09:35:29AM +0000, Catalin Marinas wrote: >> On Fri, Nov 27, 2015 at 11:12:28AM +0300, Andrey Ryabinin wrote: >> > On 11/26/2015 07:40 PM, Ard Biesheuvel wrote: >> > > On 26 November 2015 at 14:14, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote: >> > >> Currently kasan assumes that shadow memory covers one or more entire PGDs. >> > >> That's not true for 16K pages + 48bit VA space, where PGDIR_SIZE is bigger >> > >> than the whole shadow memory. >> > >> >> > >> This patch tries to fix that case. >> > >> clear_page_tables() is a new replacement of clear_pgs(). Instead of always >> > >> clearing pgds it clears top level page table entries that entirely belongs >> > >> to shadow memory. >> > >> In addition to 'tmp_pg_dir' we now have 'tmp_pud' which is used to store >> > >> puds that now might be cleared by clear_page_tables. >> > >> >> > >> Reported-by: Suzuki K. Poulose <Suzuki.Poulose@xxxxxxx> >> > >> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> >> > > >> > > I would argue that the Kasan code is complicated enough, and we should >> > > avoid complicating it even further for a configuration that is highly >> > > theoretical in nature. >> > > >> > > In a 16k configuration, the 4th level only adds a single bit of VA >> > > space (which is, as I understand it, exactly the issue you need to >> > > address here since the top level page table has only 2 entries and >> > > hence does not divide by 8 cleanly), which means you are better off >> > > using 3 levels unless you *really* need more than 128 TB of VA space. >> > > >> > > So can't we just live with the limitation, and keep the current code? >> > >> > No objections from my side. Let's keep the current code. >> >> Ard had a good point, so fine by me as well. > > Ok, so obvious follow-up question: why do we even support 48-bit + 16k > pages in the kernel? Either it's useful, and we make things work with it, > or it's not and we can drop it (or, at least, hide it behind EXPERT like > we do for 36-bit). > So there's 10 kinds of features in the world, useful ones and !useful ones? :-) I think 48-bit/16k is somewhat useful, and I think we should support it. But I also think we should be pragmatic, and not go out of our way to support the combinatorial expansion of all niche features enabled together. I think it is perfectly fine to limit kasan support to configurations whose top level translation table divides by 8 cleanly (which only excludes 16k/48-bit anyway) However, I think it deserves being hidden behind CONFIG_EXPERT more than 36-bit/16k does. -- Ard. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>