From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id 65D2D6B000A for ; Tue, 26 Jun 2018 08:10:31 -0400 (EDT) Received: by mail-oi0-f72.google.com with SMTP id c23-v6so11797749oiy.3 for ; Tue, 26 Jun 2018 05:10:31 -0700 (PDT) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id f88-v6si462443otf.158.2018.06.26.05.10.29 for ; Tue, 26 Jun 2018 05:10:30 -0700 (PDT) Date: Tue, 26 Jun 2018 13:10:26 +0100 From: Mark Rutland Subject: Re: Calling vmalloc_to_page() on ioremap memory? Message-ID: <20180626121025.xo2pgskpry2fqrpa@lakrids.cambridge.arm.com> References: <20180625160040.di75264empbcf6xz@lakrids.cambridge.arm.com> <20180625162728.qkkbzjgqebgh2fuu@lakrids.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Alexander Potapenko Cc: Ard Biesheuvel , Andrew Morton , Linux Memory Management List On Tue, Jun 26, 2018 at 12:00:00PM +0200, Alexander Potapenko wrote: > On Mon, Jun 25, 2018 at 6:27 PM Mark Rutland wrote: > > > > On Mon, Jun 25, 2018 at 06:24:57PM +0200, Alexander Potapenko wrote: > > > On Mon, Jun 25, 2018 at 6:00 PM Mark Rutland wrote: > > > > > > > > On Mon, Jun 25, 2018 at 04:59:23PM +0200, Alexander Potapenko wrote: > > > > > Hi Ard, Mark, Andrew and others, > > > > > > > > > > AFAIU, commit 029c54b09599573015a5c18dbe59cbdf42742237 ("mm/vmalloc.c: > > > > > huge-vmap: fail gracefully on unexpected huge vmap mappings") was > > > > > supposed to make vmalloc_to_page() return NULL for pointers not > > > > > returned by vmalloc(). > > > > > > > > It's a little more subtle than that -- avoiding an edge case where we > > > > unexpectedly hit huge mappings, rather than determining whether an > > > > address same from vmalloc(). > > > Ok, but anyway, acpi_os_ioremap() creates a huge page mapping via > > > __ioremap_caller() (see > > > https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/ioremap.c#L133) > > > Shouldn't these checks detect that as well? > > > > It should catch such mappings, yes. > > > > > > > For memory error detection purposes I'm trying to map the addresses > > > > > from the vmalloc range to valid struct pages, or at least make sure > > > > > there's no struct page for a given address. > > > > > Looking up the vmap_area_root rbtree isn't an option, as this must be > > > > > done from instrumented code, including interrupt handlers. > > > > > > > > I'm not sure how you can do this without looking at VMAs. > > > > > > > > In general, the vmalloc area can contain addresses which are not memory, > > > > and this cannot be detremined from the address alone. > > > I thought this was exactly what vmalloc_to_page() did, but apparently no. > > > > > > > You *might* be able to get away with pfn_valid(vmalloc_to_pfn(x)), but > > > > IIRC there's some disagreement on the precise meaning of pfn_valid(), so > > > > that might just tell you that the address happens to fall close to some > > > > valid memory. > > > This appears to work, at least for ACPI mappings. I'll check other cases though. > > > Thank you! > pfn_valid(vmalloc_to_pfn(x)) works for me, so I'll stick to this > solution for now. Thanks again! > > But just to clarify, should vmalloc_to_page() return NULL for a huge > mapping returned by __ioremap_caller()? It will not always do so. It *may* return NULL, or it may return a potentially invalid pointer to struct page. > Your answer and that of Ard seem to be contradictory. > Maybe it's a good idea to add the pfn_valid() check to > vmalloc_to_page() just to be sure? Perhaps, though it really depends on the intended use case of vmalloc_to_page(). Thanks, Mark.