linux-kernel - Re: [RFC PATCH 0/2] Add hstate parameter to huge_pte

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Message-ID: <87efxmhqar.fsf@e105922-lin.cambridge.arm.com>
Date:   Fri, 24 Mar 2017 11:23:08 +0000
From:   Punit Agrawal <punit.agrawal@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tyler Baicar <tbaicar@...eaurora.org>
Subject: Re: [RFC PATCH 0/2] Add hstate parameter to huge_pte_offset()

"Kirill A. Shutemov" <kirill@...temov.name> writes:

> On Thu, Mar 23, 2017 at 01:55:27PM -0700, Mike Kravetz wrote:
>> On 03/23/2017 05:58 AM, Punit Agrawal wrote:
>> > On architectures that support hugepages composed of contiguous pte as
>> > well as block entries at the same level in the page table,
>> > huge_pte_offset() is not able to determine the right offset to return
>> > when it encounters a swap entry (which is used to mark poisoned as
>> > well as migrated pages in the page table).
>> > 
>> > huge_pte_offset() needs to know the size of the hugepage at the
>> > requested address to determine the offset to return - the current
>> > entry or the first entry of a set of contiguous hugepages. This came
>> > up while enabling support for memory failure handling on arm64[0].
>> > 
>> > Patch 1 adds a hstate parameter to huge_pte_offset() to provide
>> > additional information about the target address. It also updates the
>> > signatures (and usage) of huge_pte_offset() for architectures that
>> > override the generic implementation. This patch has been compile
>> > tested on ia64 and x86.
>> 
>> I haven't looked at the performance implications of making huge_pte_offset
>> just a little slower.  But, I think you can get hstate from the parameters
>> passed today.
>> 
>> vma = find_vma(mm, addr);
>> h = hstate_vma(vma);
>
> It's better to avoid find_vma() in fast(?) path if possible. So passing it
> down is probably better.

Also most call sites of huge_pte_offset() already have the hstate (or
the vma) readily available. So adding overhead feels unnecessary.

I agree that merging the patch will need some co-ordination but lets at
the least give it a shot.