From mboxrd@z Thu Jan 1 00:00:00 1970 From: jonathan.austin@arm.com (Jonathan Austin) Date: Fri, 10 Aug 2012 18:43:56 +0100 Subject: Why the region area don't decrease 1 in function sanity_check_meminfo? In-Reply-To: <20120810153705.GZ18957@n2100.arm.linux.org.uk> References: <5025253D.3010007@arm.com> <20120810153705.GZ18957@n2100.arm.linux.org.uk> Message-ID: <5025485C.80407@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Russell, On 10/08/12 16:37, Russell King - ARM Linux wrote: >> With !HIGHMEM, sanity_check_meminfo checks for banks that completely or >> partially overlap the vmalloc region. The check for partial overlap checks >> __va(bank->start + bank->size) > vmalloc_min, but the last address of the >> bank is (bank->start + bank->size -1). > > Erm. > > Let's say you have a bank at 0x80000000, which maps to 0xc0000000 virtual. > This is 512MB in size (so it's last byte address is 0x9fffffff). That > places it at at 0xdfffffff. Now, let's say vmalloc_min is 0xe0000000. > > "bank->start + bank->size" would be 0xa0000000, right ? > > So, "__va(bank->start + bank->size)" would be 0xe0000000. Yep, except in our (hypothetical?) case below... > > And "0xe0000000 > vmalloc_min" would be false. > Yup... My commit message is wrong. Sorry. The linear case is fine... >> However, theoretically, if using using SPARSEMEM in a situation where the >> physical to virtual address conversion is not monotonic increasing, the >> incorrect test could result in a bank not being truncated when it should be. > > Right, so what you're actually talking about is a non-linear translation > by __va() and friends. In that case, what you actually need is: > > (__va(bank->start + bank->size - 1) + 1) > vmalloc_min > or > __va(bank->start + bank->size - 1) >= vmalloc_min > > and not > > __va(bank->start + bank->size - 1) > vmalloc_min > Agreed. I prefer the second form, and there seems to be a precedent for ">=" further up in sanity_check_meminfo, too. If you think it is worth fixing for this case, I'll make the fixes, ensure the commit message is !wrong and submit this to the patch-system. Jonny