From: Catalin Marinas <catalin.marinas@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org,
will@kernel.org, mark.rutland@arm.com, mhocko@suse.com,
ira.weiny@intel.com, david@redhat.com, cai@lca.pw,
logang@deltatee.com, cpandya@codeaurora.org,
arunks@codeaurora.org, dan.j.williams@intel.com,
mgorman@techsingularity.net, osalvador@suse.de,
ard.biesheuvel@arm.com, steve.capper@arm.com, broonie@kernel.org,
valentin.schneider@arm.com, Robin.Murphy@arm.com,
steven.price@arm.com, suzuki.poulose@arm.com
Subject: Re: [PATCH V7 3/3] arm64/mm: Enable memory hot remove
Date: Tue, 17 Sep 2019 16:08:53 +0100 [thread overview]
Message-ID: <20190917150852.GC7305@arrakis.emea.arm.com> (raw)
In-Reply-To: <a1962cde-b4df-e4a0-de61-252c0d0a25b2@arm.com>
On Tue, Sep 17, 2019 at 10:06:11AM +0530, Anshuman Khandual wrote:
> On 09/13/2019 03:39 PM, Catalin Marinas wrote:
> > On Fri, Sep 13, 2019 at 11:28:01AM +0530, Anshuman Khandual wrote:
> >> The problem (race) is not because of the inability to deal with partially
> >> filled table. We can handle that correctly as explained below [1]. The
> >> problem is with inadequate kernel page table locking during vmalloc()
> >> which might be accessing intermediate kernel page table pointers which is
> >> being freed with free_empty_tables() concurrently. Hence we cannot free
> >> any page table page which can ever have entries from vmalloc() range.
> >
> > The way you deal with the partially filled table in this patch is to
> > avoid freeing if there is a non-empty entry (!p*d_none()). This is what
> > causes the race with vmalloc. If you simply avoid freeing a pmd page,
> > for example, if the range floor/ceiling is not aligned to PUD_SIZE,
> > irrespective of whether the other entries are empty or not, you
> > shouldn't have this problem. You do free the pte page if the range is
[...]
> > We may have some pgtable pages not freed at both ends of the range
> > (maximum 6 in total) but I don't really see this an issue. They could be
> > reused if something else gets mapped in that range.
>
> I assume that the number 6 for maximum page possibility came from
>
> (floor edge + ceiling edge) * (PTE table + PMD table + PUD table)
Yes.
> >> Though not completely sure, whether I really understood the suggestion above
> >> with respect to the floor-ceiling mechanism as in free_pgd_range(). Are you
> >> suggesting that we should only attempt to free up those vmemmap range page
> >> table pages which *definitely* could never overlap with vmalloc by working
> >> on a modified (i.e cut down with floor-ceiling while avoiding vmalloc range
> >> at each level) vmemmap range instead ?
> >
> > You can ignore the overlap check altogether, only free the page tables
> > with floor/ceiling set to the start/size passed to arch_remove_memory()
> > and vmemmap_free().
>
> Wondering if it will be better to use [VMEMMAP_START - VMEMMAP_END] and
> [PAGE_OFFSET - PAGE_END] as floor/ceiling respectively with vmemmap_free()
> and arch_remove_memory(). Not only it is safe to free all page table pages
> which span over these maximum possible mapping range but also it reduces
> the risk for alignment related wastage.
That's indeed better. You pass the floor/ceiling as the enclosing range
and start/end as the actual range to unmap is. We avoid the potential
"leak" around the edges when falling within the floor/ceiling range (I
think that's close to what free_pgd_range() does).
> >> This can be one restrictive version of the function
> >> free_empty_tables() called in case there is an overlap. So we will
> >> maintain two versions for free_empty_tables(). Please correct me if
> >> any the above assumptions or understanding is wrong.
> >
> > I'd rather have a single version of free_empty_tables(). As I said
> > above, the only downside is that a partially filled pgtable page would
> > not be freed even though the other entries are empty.
>
> Sure. Also practically the limitation will be applicable only for vmemmap
> mapping but not for linear mappings where the chances of overlap might be
> negligible as it covers half kernel virtual address space.
If you have a common set of functions, it doesn't heart to pass the
correct floor/ceiling in both cases.
--
Catalin
prev parent reply other threads:[~2019-09-17 15:09 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-03 9:45 [PATCH V7 0/3] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-09-03 9:45 ` [PATCH V7 1/3] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory() Anshuman Khandual
2019-09-04 8:16 ` David Hildenbrand
2019-09-05 4:27 ` Anshuman Khandual
2019-09-16 1:44 ` Balbir Singh
2019-09-18 9:28 ` Anshuman Khandual
2019-09-03 9:45 ` [PATCH V7 2/3] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump Anshuman Khandual
2019-09-15 2:35 ` Balbir Singh
2019-09-18 9:12 ` Anshuman Khandual
2019-09-03 9:45 ` [PATCH V7 3/3] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-09-10 16:17 ` Catalin Marinas
2019-09-11 10:31 ` David Hildenbrand
2019-09-12 4:28 ` Anshuman Khandual
2019-09-12 8:37 ` Anshuman Khandual
2019-09-12 20:15 ` Catalin Marinas
2019-09-13 5:58 ` Anshuman Khandual
2019-09-13 10:09 ` Catalin Marinas
2019-09-17 4:36 ` Anshuman Khandual
2019-09-17 15:08 ` Catalin Marinas [this message]
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