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From: "joro@8bytes.org" <joro@8bytes.org>
To: "Kani, Toshi" <toshi.kani@hpe.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"bp@suse.de" <bp@suse.de>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"guohanjun@huawei.com" <guohanjun@huawei.com>,
	"willy@infradead.org" <willy@infradead.org>,
	"wxf.wang@hisilicon.com" <wxf.wang@hisilicon.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.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 19:23:28 +0200	[thread overview]
Message-ID: <20180426172327.GQ15462@8bytes.org> (raw)
In-Reply-To: <1524759629.2693.465.camel@hpe.com>

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.

> > 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.


Regards,

	Joerg

  reply	other threads:[~2018-04-26 17:23 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 [this message]
2018-04-26 17:49         ` Kani, Toshi
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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