linux-kernel - Re: [PATCH RFC v3 11/35] mm: Allow an arch to hook into folio allocation when VMA is known

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Message-ID: <7612b843-cd31-4917-87c0-c26802c5bef2@arm.com>
Date: Wed, 31 Jan 2024 12:23:51 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Alexandru Elisei <alexandru.elisei@....com>
Cc: catalin.marinas@....com, will@...nel.org, oliver.upton@...ux.dev,
 maz@...nel.org, james.morse@....com, suzuki.poulose@....com,
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Subject: Re: [PATCH RFC v3 11/35] mm: Allow an arch to hook into folio
 allocation when VMA is known



On 1/30/24 17:04, Alexandru Elisei wrote:
> Hi,
> 
> On Tue, Jan 30, 2024 at 03:25:20PM +0530, Anshuman Khandual wrote:
>>
>> On 1/25/24 22:12, Alexandru Elisei wrote:
>>> arm64 uses VM_HIGH_ARCH_0 and VM_HIGH_ARCH_1 for enabling MTE for a VMA.
>>> When VM_HIGH_ARCH_0, which arm64 renames to VM_MTE, is set for a VMA, and
>>> the gfp flag __GFP_ZERO is present, the __GFP_ZEROTAGS gfp flag also gets
>>> set in vma_alloc_zeroed_movable_folio().
>>>
>>> Expand this to be more generic by adding an arch hook that modifes the gfp
>>> flags for an allocation when the VMA is known.
>>>
>>> Note that __GFP_ZEROTAGS is ignored by the page allocator unless __GFP_ZERO
>>> is also set; from that point of view, the current behaviour is unchanged,
>>> even though the arm64 flag is set in more places.  When arm64 will have
>>> support to reuse the tag storage for data allocation, the uses of the
>>> __GFP_ZEROTAGS flag will be expanded to instruct the page allocator to try
>>> to reserve the corresponding tag storage for the pages being allocated.
>> Right but how will pushing __GFP_ZEROTAGS addition into gfp_t flags further
>> down via a new arch call back i.e arch_calc_vma_gfp() while still maintaining
>> (vma->vm_flags & VM_MTE) conditionality improve the current scenario. Because
> I'm afraid I don't follow you.

I was just asking whether the overall scope of __GFP_ZEROTAGS flag is being
increased to cover more core MM paths through this patch. I think you have
already answered that below.

> 
>> the page allocator could have still analyzed alloc flags for __GFP_ZEROTAGS
>> for any additional stuff.
>>
>> OR this just adds some new core MM paths to get __GFP_ZEROTAGS which was not
>> the case earlier via this call back.
> Before this patch: vma_alloc_zeroed_movable_folio() sets __GFP_ZEROTAGS.
> After this patch: vma_alloc_folio() sets __GFP_ZEROTAGS.

Understood.

> 
> This patch is about adding __GFP_ZEROTAGS for more callers.

Right, I guess that is the real motivation for this patch. But just wondering
does this cover all possible anon fault paths for converting given vma_flag's
VM_MTE flag into page alloc flag __GFP_ZEROTAGS ? Aren't there any other file
besides (mm/shmem.c) which needs to be changed to include arch_calc_vma_gfp() ?