From: Peter Xu <peterx@redhat.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v2] mm/hugetlb: Fix calculation of adjust_range_if_pmd_sharing_possible
Date: Thu, 30 Jul 2020 19:26:56 -0400 [thread overview]
Message-ID: <20200730232656.GE3649@xz-x1.hitronhub.home> (raw)
In-Reply-To: <4680014a-a328-b0c2-dc86-8c1eb4556f69@oracle.com>
Hi, Mike,
On Thu, Jul 30, 2020 at 02:49:18PM -0700, Mike Kravetz wrote:
> On 7/30/20 1:16 PM, Peter Xu wrote:
> > This is found by code observation only.
> >
> > Firstly, the worst case scenario should assume the whole range was covered by
> > pmd sharing. The old algorithm might not work as expected for ranges
> > like (1g-2m, 1g+2m), where the adjusted range should be (0, 1g+2m) but the
> > expected range should be (0, 2g).
> >
> > Since at it, remove the loop since it should not be required. With that, the
> > new code should be faster too when the invalidating range is huge.
>
> Thanks Peter!
>
> That is certainly much simpler than the loop in current code. You say there
> are instances where old code 'might not work' for ranges like (1g-2m, 1g+2m).
> Not sure I understand what you mean by adjusted and expected ranges in the
> message. Both are possible 'adjusted' ranges depending on vma size.
>
> Just trying to figure out if there is an actual problem in the existing code
> that needs to be fixed in stable. I think the existing code is correct, just
> inefficient.
Thanks for the quick review!
I'm not sure whether that will cause a real problem, but iiuc in my previous
example of (1g-2m, 1g+2m) in the commit message, the old code will extend the
range to (0, 1g+2m). In this case, if unluckily the (1g, 2g) range is a pud
with shared pmd, then imho we face the risk of partial tlb flushing with the
old code, because it will only flush tlb for range (0, 1g+2m) but not (0, 2g).
If that's the case, maybe it worths cc stable.
Anyway, I'd like to double confirm with you in case I missed something.
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2020-07-30 23:27 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-30 20:16 [PATCH v2] mm/hugetlb: Fix calculation of adjust_range_if_pmd_sharing_possible Peter Xu
2020-07-30 21:49 ` Mike Kravetz
2020-07-30 23:26 ` Peter Xu [this message]
2020-07-31 0:28 ` Mike Kravetz
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