linux-kernel - Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

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Message-ID: <YxJot6JbFl0HnXq3@xz-m1.local>
Date:   Fri, 2 Sep 2022 16:33:59 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     david@...hat.com, kirill.shutemov@...ux.intel.com,
        jhubbard@...dia.com, jgg@...dia.com, hughd@...gle.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

On Fri, Sep 02, 2022 at 10:45:20AM -0700, Yang Shi wrote:
> > How about another patch to remove the ppc impl too?  Then it can be a two
> > patches series.
> 
> BTW, I don't think we could remove the ppc implementation since it is
> different from the generic pmdp_collapse_flush(), particularly for the
> hash part IIUC.
> 
> The generic version calls flush_tlb_range() -> hash__flush_tlb_range()
> for hash, but the hash call is actually no-op. The ppc version calls
> hash__pmdp_collapse_flush() -> flush_tlb_pmd_range(), which does
> something useful.

One thing I found interesting (and also a bit confused..) is that the ppc
code used the name flush_tlb_pmd_range() to "flush tlb range in pte level",
which is kind of against the tlb API design..

The generic tlb API has a very close function called flush_pmd_tlb_range()
which is only used to do pmd-level flushing, while here the ppc version of
flush_tlb_pmd_range() is actually flush_tlb_range() in the generic API.

Agreed that it may worth having a look from ppc developers.

-- 
Peter Xu