linux-kernel - Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast

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Message-ID: <YxD00K1lv151X/eq@xz-m1.local>
Date:   Thu, 1 Sep 2022 14:07:12 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Sasha Levin <sasha.levin@...cle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jerome Marchand <jmarchan@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast

On Thu, Sep 01, 2022 at 10:50:48AM -0700, Yang Shi wrote:
> Yeah, because THP collapse does copy the data before clearing pte. If
> we want to remove pmdp_collapse_flush() by just clearing pmd, we
> should clear *AND* flush pte before copying the data IIRC.

Yes tlb flush is still needed.  IIUC the generic pmdp_collapse_flush() will
still be working (with the pte level flushing there) but it should just
start to work for all archs, so potentially we could drop the arch-specific
pmdp_collapse_flush()s, mostly the ppc impl.

This also reminded me that the s390 version of pmdp_collapse_flush() is a
bit weird, since it doesn't even have the tlb flush there.  I feel like
it's broken but I can't really tell whether something I've overlooked.
Worth an eye on.

-- 
Peter Xu