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Message-ID: <1527610139.14039.58.camel@hpe.com>
Date: Tue, 29 May 2018 16:10:24 +0000
From: "Kani, Toshi" <toshi.kani@....com>
To: "tglx@...utronix.de" <tglx@...utronix.de>,
"joro@...tes.org" <joro@...tes.org>,
"mingo@...hat.com" <mingo@...hat.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"stable@...r.kernel.org" <stable@...r.kernel.org>,
"x86@...nel.org" <x86@...nel.org>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"hpa@...or.com" <hpa@...or.com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"cpandya@...eaurora.org" <cpandya@...eaurora.org>,
"Hocko, Michal" <MHocko@...e.com>
Subject: Re: [PATCH v3 3/3] x86/mm: add TLB purge to free pmd/pte page
interfaces
On Tue, 2018-05-29 at 16:44 +0200, Joerg Roedel wrote:
> On Wed, May 16, 2018 at 05:32:07PM -0600, Toshi Kani wrote:
> > pmd = (pmd_t *)pud_page_vaddr(*pud);
> > + pmd_sv = (pmd_t *)__get_free_page(GFP_KERNEL);
> > + if (!pmd_sv)
> > + return 0;
>
> So your code still needs to allocate a full page where a simple
> list_head on the stack would do the same job.
Can you explain why you think allocating a page here is a major problem?
As I explained before, pud_free_pmd_page() covers an extremely rare case
which I could not even hit with a huge number of ioremap() calls until
I instrumented alloc_vmap_area() to force this case to happen. I do not
think pages should be listed for such a rare case.
> Ingo, Thomas, can you please just revert the original broken patch for
> now until there is proper fix?
If we just revert, please apply patch 1/3 first. This patch address the
BUG_ON issue on PAE. This is a real issue that needs a fix ASAP.
The page-directory cache issue on x64, which is addressed by patch 3/3,
is a theoretical issue that I could not hit by putting ioremap() calls
into a loop for a whole day. Nobody hit this issue, either.
The simple revert patch Joerg posted a while ago causes
pmd_free_pte_page() to fail on x64. This causes multiple pmd mappings
to fall into pte mappings on my test systems. This can be seen as a
degradation, and I am afraid that it is more harmful than good.
Thanks,
-Toshi
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