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

From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH RFC v3 11/35] mm: Allow an arch to hook into folio allocation when VMA is known
Date: Wed, 31 Jan 2024 12:22:05 +0000	[thread overview]
Message-ID: <Zbo7bVq822iphFtc@raptor> (raw)
In-Reply-To: <7612b843-cd31-4917-87c0-c26802c5bef2@arm.com>

Hi,

On Wed, Jan 31, 2024 at 12:23:51PM +0530, Anshuman Khandual wrote:
> 
> 
> 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() ?

My thoughts exactly. I went through most of the fault handling code, and
from the code I read, all the allocation were executed with
vma_alloc_folio() or by shmem.

That's not to say there's no scope for improvment, there definitely is, but
since having __GFP_ZEROTAGS isn't necessary for correctness (but it's very
useful for performance, since it can avoid a page fault and a page
migration) and this series is an RFC I settled on changing only the above,
since KVM support for dynamic tag storage also benefits from this change.

The series is very big already, I wanted to settle on an approach that is
acceptable for upstreaming before thinking too much about performance.

Thanks,
Alex