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From: "Kani, Toshi" <toshi.kani@hpe.com>
To: "joro@8bytes.org" <joro@8bytes.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"bp@suse.de" <bp@suse.de>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"guohanjun@huawei.com" <guohanjun@huawei.com>,
	"wxf.wang@hisilicon.com" <wxf.wang@hisilicon.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"willy@infradead.org" <willy@infradead.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"will.deacon@arm.com" <will.deacon@arm.com>,
	"Hocko, Michal" <MHocko@suse.com>,
	"cpandya@codeaurora.org" <cpandya@codeaurora.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces
Date: Thu, 26 Apr 2018 17:49:58 +0000	[thread overview]
Message-ID: <1524764948.2693.478.camel@hpe.com> (raw)
In-Reply-To: <20180426172327.GQ15462@8bytes.org>

On Thu, 2018-04-26 at 19:23 +0200, joro@8bytes.org wrote:
> On Thu, Apr 26, 2018 at 04:21:19PM +0000, Kani, Toshi wrote:
> > All pages under the pmd had been unmapped and then lazy TLB purged with
> > INVLPG before coming to this code path.  Speculation is not allowed to
> > pages without mapping.
> 
> CPUs have not only TLBs, but also page-walk caches which cache
> intermediary results of page-table walks and which is flushed together
> with the TLB.
> 
> So the PMD entry you clear can still be in a page-walk cache and this
> needs to be flushed too before you can free the PTE page. Otherwise
> page-walks might still go to the page you just freed. That is especially
> bad when the page is already reallocated and filled with other data.

I do not understand why we need to flush processor caches here. x86
processor caches are coherent with MESI.  So, clearing an PMD entry
modifies a cache entry on the processor associated with the address,
which in turn invalidates all stale cache entries on other processors.

> > > Further this needs synchronization with other page-tables in the system
> > > when the kernel PMDs are not shared between processes. In x86-32 with
> > > PAE this causes a BUG_ON() being triggered at arch/x86/mm/fault.c:268
> > > because the page-tables are not correctly synchronized.
> > 
> > I think this is an issue with pmd mapping support on x86-32-PAE, not
> > with this patch.  I think the code needed to be updated to sync at the
> > pud level.
> 
> It is an issue with this patch, because this patch is for x86 and on x86
> every change to the kernel page-tables potentially needs to by
> synchronized to the other page-tables. And this patch doesn't implement
> it, which triggers a BUG_ON() under certain conditions.

The issue was introduced when pmd mapping support was added on x86/32,
which was made prior to this patch.

Thanks,
-Toshi

  reply	other threads:[~2018-04-26 17:50 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-14 18:01 [PATCH v2 0/2] fix memory leak / panic in ioremap huge pages Toshi Kani
2018-03-14 18:01 ` [PATCH v2 1/2] mm/vmalloc: Add interfaces to free unmapped page table Toshi Kani
2018-03-14 22:38   ` Andrew Morton
2018-03-15 14:27     ` Kani, Toshi
2018-03-14 18:01 ` [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces Toshi Kani
2018-03-15  7:39   ` Chintan Pandya
2018-03-15 14:51     ` Kani, Toshi
2018-04-26 14:19   ` Joerg Roedel
2018-04-26 16:21     ` Kani, Toshi
2018-04-26 17:23       ` joro
2018-04-26 17:49         ` Kani, Toshi [this message]
2018-04-26 20:07           ` joro
2018-04-26 22:30             ` Kani, Toshi
2018-04-27  7:37               ` joro
2018-04-27 11:39                 ` Michal Hocko
2018-04-27 11:46                   ` joro
2018-04-27 11:52                 ` Chintan Pandya
2018-04-27 12:48                   ` joro
2018-04-27 13:42                     ` Chintan Pandya
2018-04-27 14:31                 ` Kani, Toshi
2018-04-28  9:02                   ` joro
2018-04-28 20:54                     ` Kani, Toshi
2018-04-30  7:30                       ` Chintan Pandya
2018-04-30 13:43                         ` Kani, Toshi

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